Their relationship was mainly for sex, but when Jess returns in Season 3 it is apparant that they stayed together. She was well aware for Trey's affection for Marissa, but still wanted him. In her own little piece of revenge, she flirts with Ryan, who respectfully turns her down.
After botching an attempted drug deal, Jess decides to leave town with Trey but remains when he is shot by Marissa. In the third season, Jess asks Ryan to help her with boyfriend troubles.