Sean is a former convict and Maria DeLuca's cousin. After being released from prison, Amy DeLuca took him in and allowed him to stay with them, despite Maria's protesting. Maria and Sean have a very tenuous relationship, and she keeps him on a very short leash, attempting to keep him from "violating house rules" like not wearing pants in the living room, and pursuing Liz Parker. Maria, being the protective best friend as always, tries with all her might to protect Liz from Sean's charms, even though he has piqued her interest from the very beginning.
Sean and Liz have immediate chemistry and get along extremely well. The first real encounter we see with Sean and Liz is when he teases her and she snaps, telling him that she has had a horrible day. He asks her to sit down and try his Aunt Amy's trick, write your problems in mustard on a burger, and then eat it. It makes all your problems go away. Liz does, writing "Grow" on the patty before biting into it. They are very slowly becoming close friends, until one night, out of nowhere, he kisses her; she doesn't pull away for quite a while. The next day, after hardly discussing the situation, Liz misses her bus and Sean sees her in need, offering to give her a ride. It is then revealed that he is legally not allowed within two blocks of Roswell High.
After Liz sees Max and Tess kiss at Prom, she takes off and runs into Sean, who taunts her jokingly until she snaps at him and tells him her life is falling apart, he then asks if she wants to go somewhere and she says yes. He breaks into the bowling alley and takes her with him, he sips a beer, and tells her how he feels about her. He then takes off his shoes and starts sliding around the lanes, but she refuses to join him.
Sean helps Maria and Amy take "George W. is an alien" t-shirts into the U.F.O. center to sell, and gets caught in a hostage situation, held by Brody Davis. Sean is stabbed while attempting to attack Brody, and get them out of the U.F.O. center, but in the end he is healed, and gets a date with Liz in return for his silence about what happened there. They go on a few dates, and things seem to be going well. When Max tells her that he's leaving, Liz breaks down and runs to Sean, making out with him for a rather long time until she get so overwhelmed she can't handle it anymore, and tells him what's going on. She apologizes over and over for doing this to him and he says that it's okay, comforting her until she's ready to take off after Max.