Very little is known of Joaquin's life. He resided on the south side of town, and some time prior to the start of the series he joined the Southside Serpents, a dangerous biker gang of criminals and thugs. It can be assumed he attended Southside High.
Unlike his unruly fellow gang members, Joaquin attended the Twilight Drive-In to enjoy the film, and he did so, until he noticed Kevin Keller, shushing at them to quite down from several feet away. With Kevin in sight, Joaquin followed him to the food stand, in which a brief confrontation took place. Moments later, the two are seen making out in a back ally, behind the drive-in. Before leaving, Kevin gave Joaquin his phone number, almost immediately, he noticed Kevin's last name was Keller, as in the sheriff's son. As to be expected, he was alarmed by the revelation, but not frightened off, in spite of the fact that he's a gang member. 
Upon learning that one of Kevin's friends had been attacked on the Andrews Construction work site, possibly by a Serpent, Joaquin agreed to assist them in gaining access to a known Serpent hang out called the Whyte Wyrm. He first reminded Kevin Archie, and Moose to play it cool, and to keep eye contact to a minimum. The plan was for them to find the guys that ambushed Moose, and call Sheriff Keller, but Archie took it upon himself to confront the potential attacker, who was a Serpent named Mustang. Joaquin warned Archie to back off, but he didn't, which led to a brief altercation that was broken up by FP Jones, the leader of the Serpents. Later that night, he joined FP at his home in the trailer park after discovering that the police had learned of their arrangement with Jason Blossom. The heat was on, so FP packed Jason's varsity jacket in a bag, and tossed it to Joaquin, to hide in a safe place as insurance, in case everything went to hell.
It was then revealed that Joaquin's relationship with Kevin was only a means to an end, they planned on using Kevin as an inside line to the Sheriff. However, Joaquin felt guilty as Kevin's feelings for him were genuine, but FP reminded him that they all had a role to play, even him.
Despite having earlier established that Jughead's 16th Birthday Party was suppose to be inner-circle only, Kevin invited Joaquin to the party, who he claimed was inner-circle because they were dating. Unbeknownst to neither Kevin nor Veronica, Joaquin eavesdropped on their conversation pertaining to her father's trial and his potential involvement in Jason Blossom's Death. As Jughead and Betty entered the house, everyone jumped out yelling surprise, then Kevin introduced Joaquin to Jughead. Then came the birthday cake, as Joaquin, Kevin, Archie, Veronica, and Ethel surrounded Jughead while he made a wish and blew out the candles. This small and quiet event quickly escalated into a full blown house party as the party was crashed by Cheryl Blossom and Chuck Clayton, along with many of their classmates.
Joaquin and Kevin did not allow this to put an end to their good time, and proceeded to make out in the kitchen, until they were interrupted by FP, who asked Joaquin to show him to the bathroom, which was merely an excuse for them to talk privately. He informed FP that Hiram Lodge's daughter was also at the party, during which she mentioned her father possibly being connected to Jason's murder. He was then ordered to find out what the Sheriff knew, and to do that through Kevin. Unperceived by either of them, they were being watched from the next house over by Alice Cooper. Downstairs, Joaquin watched quietly as a devious gamed of "Secrets & Sins", took place, however, the game quickly turned violent between Jughead and Chuck Clayton, which resulted in FP bringing an end to the party.
Joaquin accompanied Kevin to Riverdale High School's Homecoming dance. After Archie and Veronica performed "Kids in America" on stage, he and Kevin slow danced, during which Kevin revealed that not only was that the first time he had brought a boyfriend to that kind of event, it was the first time he ever really had a boyfriend. Most of his past relationships consisted of brief hook ups with closeted guys. They were then interrupted by Jughead, who was looking for Betty. From a distance, Joaquin immediately spotted Sheriff Keller as he entered the dance and approached Mayor McCoy, leaving Joaquin in an uneasy state as he slowly backed away.
Two days after FP's arrest for the murder of Jason Blossom, Joaquin was invited over to Archie's house, where Kevin, Veronica and Archie asked him about the call that he had received from FP. Joaquin claimed the call was simply a warning to lay low, but Veronica wasn't buying it as no one uses their only call so frivolously. As the sheriff's son, Kevin asked him if he knew anything. Hesitantly, Joaquin said he had always assumed that FP killed Jason, he never saw FP pull the trigger but on July 11th, he got a call in the middle of the night from FP, who needed his help with a clean up job at the Whyte Wyrm. The basement of the bar was off limits to everyone, but that's where he met FP, and that's when he saw Jason's body laying on the ground.
He was terrified, so he never asked FP if he killed Jason, Joaquin only helped with disposing of the body and cleaning up the mess left behind. Veronica stated that FP had done jobs for her father, Hiram Lodge, in the past, she asked if he thought that killing Jason was one of them, but he wasn't sure. There was one time that he had overheard a conversation between FP and Mustang, the only other guy who know about the clean up, and the Serpent that Archie had previously gotten into a fight with. They were talking about some rich guy, but he wasn't entirely sure who. They threatened to turn him in to Sheriff Keller if he didn't take them to Mustang's place of residence, which was the City Centre Motor Hotel.
Inside the motel room, they found Mustang dead in the bathtub, it appeared that he had overdosed, but Joaquin couldn't stick around for when the police arrived, so he immediately exited, with Kevin right behind. Left without too many options, Joaquin decided it was best to leave town. He left on a bus headed for San Junipero, but not before sharing one last passionate kiss with Kevin, and informing him about the contingency plan between him and FP, which involved a duffel bag with Jason's varsity jacket inside.
Joaquin has a slim build, with tan skin, medium-length black hair, and very light blue eyes. At first glance, he comes off as grungy and dangerous, the fact that he is affiliated with the Southside Serpents, doesn't help. Apparently, when joining the gang, members are required to tattoo the gang's symbol of a serpent on their body. He wears clothing similar to many of the Serpents and has a snake tattoo on his right wrist.
Admittedly, Joaquin is a member of the Southside Serpents, however, while he can be intimidating, he does not appear to be as ruthless and savage as his fellow gang members, but merely a man of few words and rather quiet. He has a conscience, as evidenced when he revealed to FP that he felt bad for stringing Kevin Keller along into thinking they had a genuine relationship, as Kevin had real feelings for him. Joaquin is also fairly calm and relaxed, even under the most dangerous of situations, like leading Kevin, Archie and Moose into a Serpent inhabited bar. It's implied that Joaquin developed true feelings for Kevin, as he professed he was going to miss Kevin as he boarded a bus to San Junipero.
- "Chapter Four: The Last Picture Show"
- "Chapter Eight: The Outsiders"
- "Chapter Ten: The Lost Weekend"
- "Chapter Eleven: To Riverdale and Back Again"
- "Chapter Twelve: Anatomy of a Murder"
- Unlike most of the other characters, Joaquin is not a character from the Archie Comics.
- Joaquin's last name was hinted to start with a "D" by his actor Rob Raco on twitter and was finally confirmed to be "DeSantos" in Chapter Twelve by Archie's mother.
- The bus Joaquin took in Chapter Twelve is to San Junipero, which is a reference to the namesake episode of the British anthology series "Black Mirror".
- Despite appearing the same age as the main characters, Joaquin isn't shown to attend Riverdale High School.
- Joaquin's sexuality is never stated on the show, despite his relationship with Kevin.