Archie could've told Weatherbee the hood was mine. But he didn't. In the Book of Reg, that makes you a top tier loyal badass. And we were talking today, and decided if you want to keep the Red Circle going, we're here for you, dude.
Reggie presumably grew up in Riverdale, where he lives with his mother, Melinda, and his father, the owner of the Luxury Car Jamboree, a local car dealership who has recently expanded his business, adding a bigger dealership on the Eastern Seaboard. Reggie has spent much of his teenage years doing odd jobs at the car dealership, building his athletic physique, and joyriding with some of the cars for sale.  Reggie's closest friend appears to be Moose Mason; they are almost always together.
The first day of school, Reggie joined Archie Andrews at his locker, hyped over Archie's new athletic build, which notably made him perfect for the football team. Archie is "ripped", said Reggie, right before asking Archie how much he benches, and if he uses performance-enhancing drugs. Archie explained that the recent change in his appearance is from working construction all summer at his father's company, Andrews Construction. This led Reggie into his next question, if Archie had sex with any older women as he was out there working on private homes, but he avoids answering.
Reggie ranted on to Moose and Chuck about Sheriff Keller bringing him in for questioning over the death of Jason Blossom. However, it would have been illogical for Reggie to kill Jason because now the football team is in need of a good quarterback. He went on to say that he should've sent the cops to Moose, as there's still a lingering question that has yet to be answered, why Moose was at Sweetwater River with Kevin Keller in the first place. They found the body, yet they weren't questioned, Reggie implied that being with the Sheriff's son gave Moose a free pass. He then directed his attention to Jughead Jones, who he referred to as a spooky, scrawny, pathetic, internet troll, which made him the perfect suspect. Reggie asked him what it was like to kill Jason, and if he did anything to the body afterward. Jughead reminded him that the correct term was necrophilia, he proceeded to ask Reggie if he could spell the word. This offended Reggie, he leaped out of his seat, towards Jughead, but Archie intervened, leading Reggie to believe that both of them kill Jason. Archie shoved Reggie, but he who shoved back a lot harder, knocking Archie to the ground, then finishing him off with a single punch. 
Reggie shared an amused look with Moose after learning that Chuck would be going on a date with Veronica Lodge. Unbeknownst at the time, the look they shared was due to the fact that Veronica would soon be added to the football team's "playbook". A book in which players of the team would list and rank each classmate they hooked up with. 
Following morning practice, Reggie was asked by Coach Clayton if he'd like to be captain of the Riverdale Bulldogs, Reggie replied that he would. The coach had put him and Archie against each one another to see who would impress him the most, within a week time span, the winner would be the new captain. Reggie believed if anyone deserved to be team captain, and wear Jason's jersey, it's him. Despite Archie's best efforts, Reggie got the better of him on every play, except for one, in which Archie managed to score on him. It wasn't long before Reggie discovered that Archie injured his hand, he insisted that he sit out on practice, but Archie refused.
When the day for the coach to pick the next captain finally arrived, it was almost too close to call, unfortunately for Reggie, Coach picked Archie. However, in an unexpected turn of events, Archie turned down the position, as he couldn't put all of himself in the team, he suggested that it be given to Reggie instead. 
Reggie and Moose, along with multiple other players on the Riverdale Bulldogs, heckled Archie, as he got on stage to audition for the 75th annual Variety Show. Archie's stage fright was so severe that he believed that he saw the players wearing werewolf masks. 
Reggie volunteered to help in the search party for Polly Cooper. They began in Eversgreen Forest, and headed East, as that's the direction Polly would have gone if she was trying to leave town undetected. That night, he joined Veronica, Kevin, and Josie, as the four of them went to a local club named the Roving Eye, where they got drunk and danced the night away.
Afterwards, Veronica explained the reason for their outing, and how everything was taken from her family once her father was arrested. Even the clothes on their backs were taken, but then her mom told her there was one thing that no one in the world could ever take from her. Reggie being the clueless jock that he is, assumed it was her trust fund, when in fact it was her name. Soon after, the club manager returned with Veronica's credit card, claiming it had been reported stolen, and that he was supposed to call the police because of it. However, Reggie reminded him that this wouldn't be the best idea as they are minors, which would subsequently result in legal troubles for the club. 
Upon the return of Polly Cooper to Riverdale High School, with Betty on one arm and Veronica on the other, Reggie commented on Polly's appearance, saying "as if I didn't already have mommy issues", which was a reference to her obvious pregnancy. He welcomed Polly back, acknowledging how good she looked. Veronica then told him to beat it, to which Reggie remarked that he would at a later time, while thinking about Polly. 
Reggie was approached at his locker by Midge Klump, who was seeking to score some Jingle Jangle for herself as well as her boyfriend, Moose. Reggie instructed Midge to lower her voice while on school property as even the slightest mention of the drug could result in serious consequences for the two of them. He then told her that he'll see what he can do. As Midge left the area, Reggie spotted Archie across the hall, at the front door of the guidance counselor's office. Why are you seeking guidance from Ms. Burble, Reggie asked, though Archie replied that she wasn't. However, Reggie didn't think it would be such a bad idea if Archie did, in fact, seek counseling considering that he looked like Reggie's nuts after football practice. Archie was having trouble with staying awake, so Reggie offered him a number of solutions, all of which consisted of unprescribed medicating with such drugs as Amphetamines, Baclofen, Modafinil. He also offered Archie some Jingle-jangle, which would keep him up for days, in more ways that one. But Reggie couldn't help but wonder why Archie was seeking "uppers". Archie explained that he wanted to be at high alert until his father's shooter is apprehended. With that in mind, Reggie agreed to supply Archie with the drugs.
That evening during class, Reggie, amongst other students, learned that Ms. Grundy last semester's music teacher, was murdered in her Greendalehome.
Later that night, Reggie planned to deliver Archie's Jingle-Jangle as planned. However, he did so in an unsuspecting and rather cruel manner; by showing up to Archie's house in a black hood, similar to that of the shooter. However, before he could even reach the door, Archie tackled him to the ground and ripped off the hood. Reggie almost immediately explained that it was a prank, he didn't expect to be attacked on sight. He then told Archie that he was losing his mind and that the last thing he needed was drugs. Reggie wondered what would have followed had he been the actual shooter. He reminded Archie that the black hooded assailant had a gun. Only a dumbass lunatic brings a baseball bat to a gun fight, Reggie explained.
Reggie ran onto the field later during practice to inform the team that Moose and Midge were at Riverdale General Hospital after Moose was shot several times late last night at lover's lane while kissing and taking Jingle Jangle. Fortunately, both Moose and Midge were alive. Moose even shielded Midge from the gunfire with his own body, saving her from any physical harm. Due to this tragic incident and Coach Clayton being at the hospital with the two students, practice would end early that evening
Shortly after cancelling practice, Reggie and several other players intended to visit Moose at the hospital, first stopping at the student lounge to see if Archie would be joining them, which he did.
Upon arriving at the hospital, Reggie and Archie joined Moose in his room, during which time Reggie stole his dessert, then asking him if his nurse was attractive. If so, he ran the risk of getting a boner during a sponge bath. While he squeezed in next to Moose on the single hospital bed, Archie began questioning him on the shooter. By Moose's account, his attacker was wearing a black hood, similar to that of the man that shot Archie's father at Pop's shoppe. Midge then interjected, describing the shooters eyes as being blank, as if there wasn't even a person behind the hood, comparable to those of Michael Myers or the devil himself.
Tired of being under attack by the hooded shooter, Archie called for a meeting to discuss how best to handle the situation. This time it was Moose and Midge, and while they survived, everyone may not be so fortunate next time around. They agreed to start a watch group to focus on Riverdale High, to operate as an extra circle of protection. Reggie took this to mean that Archie wanted them to take action by heading into and seeking out their own version of justice. However, this was not the case. While that was Archie's first thought, he came up with a better plan that excluded the use of violence and weapons; a patrol. And extra sets of eyes and ears for the police. Should they encounter the shooter, Archie told them to call Sheriff Keller. They were watchdogs, not vigilantes. As for an official name, The Red Circle was specifically chosen by Archie. From that point forward, they would protect themselves. The Red Circle then took to the streets, passing out flyers to residents who may feel scared or need protection.
The following night, Reggie waited in his car for Archie to arrive when he noticed the wooden baseball bat in Archie's hand, reminding him of his previous statement about not being vigilantes and having no need for weapons. Archie explained that he's had the bat since he was ten. In response, Reggie told Archie to look in the backseat, where he had a pipe and a lug wrench on standby in case the time came when he would have to use them. Reggie then pulled off, and together, the two of them began their patrol of the streets. It wasn't long before they received their first call; the caller being Ethel Muggs. She claimed that she was being stalked by someone in a large van on Hastings street. He and Archie soon arrived on scene to Ethel hiding in the cornfield and the van that Ethel previously described, speeding off.
Disband the Red Circle?
When the Riverdale Bulldogs were suspended due to the vigilantes' activity, the Red Circle met at the school's locker room, where Reggie tried to dissuade Archie's from his vendetta against the Black Hood, suggesting him to sign the headmaster's paper to revoke the suspension. When Archie refused, Reggie unleashed his anger against school's furniture, kicking a locker. Reggie was very much at odds with Archie's recklessness to provoke the Black Hood, since the killer could come after them, and when Archie suggested going to the Southside for an open declaration of war, Reggie called himself out of the group, taking with him the other Bulldogs from the meeting, leaving Archie and Doiley alone with their dangerous plan.
Next evening, Reggie went to Andrews house with the rest of the Red Circle members and pizza boxes, just in time to cool off an argument between Archie and Veronica. Since Archie hadn't betrayed him with Mr. Weatherbee and Sheriff Keller by keeping it secret that the black hood found at school was the one Reggie used to prank him, Reggie reconsidered his decision to quit their friendship, stating that Archie's was a loyal friend and that the group decided to support Archie's personal fight against the Black Hood. The reconciliation was suddenly interrupted by some Southside Serpents led by Sweet Pea, seeking revenge against Archie after he threatened them at their own turf. Reggie steadily stepped in, stating that Sweet Pea had a bad timing because "Bulldogs ate snakes for lunch". The two sides went out for a rumble, which degenerated quickly until Veronica put an end to it with a gunshot towards the sky, pushing the Serpents to flee. Reggie, then, promptly helped Dilton, who had been stabbed in the leg during the fight. Headed to the hospital, Reggie told him to lie about the reason he got injured. 
When Alice Cooper gathered in her living room the parents of those who were at Nick St. Clair's party, Reggie was standing beside his mom who advised him not to say anything without a lawyer. Unfortunately for her, Reggie already spilled out he was the one who gave Jingle Jangle to Nick. Veronica came to his rescue, blaming Nick for asking it but Reggie was still forced to admit that his dealer was from the Southside. Grounded, Reggie and the gang were sent by Mayor McCoy to picking up garbage at Pickens Park.
When there, Reggie started hitting on Josie who wasn't helping at all. Reggie told her that although she sees herself as Beyoncé, that day she was just a commoner like any of them. The two shortly flirted until Veronica interrupted them to ask for a private chat with Reggie. Once far from prying ears, Veronica asked him the name of the dealer, saying she wouldn't take a "No" for an answer.
Reggie was with the rest of the group exchanging Christmas gifts in the school cafeteria, making jokes as usual. When Veronica received a gift certificate for a couple's massage from Josie, who quickly suggested going with her mom, implicitly referring to Archie and Veronica's recent breakup, Reggie volunteered for a massage with Hermione. The exchange of gifts was interrupted when Midge and Moose joined the group. Reggie immediately ran to hug Moose, who had not yet recovered from the shooting, supporting himself with the aid of a crutch. 
Next day, when the Southsiders students assigned to Riverdale High arrived at school, Reggie walked alongside Cheryl, Bulldogs and River Vixens to spoil Lodge's welcoming committee. The tension between the two sides became increasingly untenable, with Cheryl provoking the Southsiders with biased remarks on how the school standards would increasingly drop due to their underachieving behavior. Archie stepped in, suggesting to put aside their Northside vs Southside differences and start over, to which Reggie replied that he doesn't speak for the Bulldogs. Reggie and Sweet Pea, then, nearly come to blows after Reggie promptly reminded Archie about the rumble with the Southside Serpents which happened a few months earlier. The heated discussion was interrupted before it could degenerate by Principal Weatherbee, inviting students to go to class. Before leaving, Reggie leaned toward Veronica, murmuring to her that "they traitors picked the wrong side".
Some time later, Principal Weatherbee harshly asked for Jughead Jones, Sweet Pea, Toni Topaz and Fangs Fogarty (who were spending time in the student lounge with Kevin, Veronica and Josie) to follow him in the corridor, inquiring them about a graffiti of the Southside Serpents' symbol drawn on the corridor floor. Reggie took advantage of it for sneaky remarks about the Serpents, and how they did nothing but deface everything. He then had a brief debate with Veronica, not grasping her dainty subtle irony about Reggie's inability to be properly sneaky; since he was showing off too much, it was evident that the graffiti had been made by the Bulldogs to put the blame on the Serpents. Nevertheless, the Mr. Weatherbee punished the Southsiders, forcing them to remove or hide any gang paraphernalia such as the leather jackets or the tattoos during school hours.
When, on the following day, Jughead showed up at school wearing his leather jacket, Reggie triggered and demanded him to remove his gang jacket. At Jughead's denial, the two began to scuffle in the student lounge and ended up in the principal's office, where Mr. Weatherbee suspended Jughead.
In the days to follow, Reggie became unwillingly accustomed to having Southside Serpents in the corridors of Riverdale High School. In particular, his well-known pride was hurt in one of his major interests: sports. Reggie, in fact, was literally thrown to the ground by Sweet Pea during a basketball game, adding more and more impatience towards the Southsiders invading his own spotlight and school. His friend Archie, then, thought well to add insult to injury, taking a break from the basketball team to join the school wrestling team. 
Reggie is a tall, attractive, and athletically built young man, his appearance is likely due to his position on the school's football team. He has straight black hair, dark brown eyes, and olive skin. His clothing choices are typically very casual, only a t-shirt and jeans.
Reggie embodies all the usual characteristics associated with the stereotypical jock. He isn't much of an intellectual and his biggest concerns seems to be football and sex, not just for himself, but his fellow teammates as well. Like most jocks, he's socially and physically fit; the typical "cool" teenager. However, it's shown that Reggie had enough consciousnesses to ask Archie to not keep hurting his injured arm. He also comforted Archie after his dad was shot. However, his compassion is short-lived; ironically, and a cruel sense of humor, he pranked Archie by wearing a homemade black hood, while coming to Archie's home. Reggie also has a strong hatred of the Southside Serpents, if not Southsiders in general, even going as far to frame them when they were transferred to Riverdale High.
The relationship between Reggie Mantle and Jughead Jones can easily be defined as a rivalry, as they have both shown a great amount of disdain for one another. This rivalry likely stems from Reggie's typical jock nature, to pick on outsiders and misfits, such as Jughead, who was subject to a lot of Reggie's bullying. Jughead and Reggie had been adversaries long before the start of the series as their dislike of one another seems well ingrained.
Reggie Mantle and Dilton Doiley are currently the only two characters to be portrayed by two actors.
Neither Reggie nor Moose Mason were cut from the football team, the way the Chuck Clayton and a number of other players were, when the scandal of Chuck's "playbook" broke, despite their names being clearly visible in the book.
In the 2015 "Reggie and Me" comic series, it is explained that this rivalry is mostly due to Betty's intrusion into Reggie and Archie's friendship when they were kids.
In the same series, Reggie has a bad to non-existent relationship with his parents who are too busy in their socialite life and they often leave him alone at home. In this iteration, his father is the publisher of the local newspaper, while his mother is a philanthropist.
In 2015 "Reggie and Me", Reggie has a crush on Midge Klump and for this reason, he has an on-going rivalry with Moose Mason, Midge's boyfriend.