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 quarter back. 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 afterwards. 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. Coach had pit 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 ever 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 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 went 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 suppose to call the police because of it. However, Reggie reminded him that this wouldn't be the best idea as they're 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 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. However, it's shown that Reggie had enough of a consciousnesses to ask Archie to not keep hurting his injured arm. He also comforted Archie after his dad was shot.
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.