Jess Mariano is the nephew of Luke Danes, and the second boyfriend of Rory Gilmore. He lived in New York with his mother[his father had abandoned them when Jess was a baby], but was sent to live with Luke in Stars Hollow in season 2, when his mother was having a lot of trouble dealing with him. He is rebellious, but is also extremely well-read, which is one of the reasons Rory develops an interest in him. He and Rory soon become friends, despite the fact that Lorelai and numerous other people tell her that he is a bad influence. He lives with Luke for a while until one night when he and Rory are in a car crash, which everyone in town except for Rory believes was his fault. Jess goes back to live with his mom. Rory skips school and comes to visit him, because they hadn't said goodbye before he left. Shortly thereafter, Jess visits Luke and asks if he can come back. Luke lets him move back in, and when Rory sees that he's back she kisses him, even though she was dating Dean at the time. She then tells him not to say anything about it.
At the beginning of season 3, he dates a girl named Shane, but he doesn't care enough about her to even learn her last name. He and Rory officially begin dating after Dean breaks up with Rory at a dance marathon because her feelings for Jess are becoming more and more obvious. They date for most of season 3. Jess's father appears in Luke's diner one night, but leaves quickly. Eventually Luke discovers that Jess has flunked out of school and won't be graduating. Luke says that Jess either has to repeat 12th grade or leave, and Jess chooses to leave. He moves out to California to live with his dad, without telling Rory; thus ending their relationship. He doesn't appear again until months later, because his mom tells him that Luke had stolen his car. Jess comes back to retrieve it, running into Rory several times. He finally tells Rory he loves her and drives away before she responds. He comes back a few month later for his mother's wedding, and later he visits Rory at Yale, telling her to run away with him. She declines, and we don't see him again until season 6.
In season 6 he comes to visit Rory[who is living with her grandparents at the time], and shows her the book he has written. After meeting her new boyfriend Logan, Jess questions Rory because she has changed so much and is no longer acting like herself, which causes Rory to rethink what she has been doing with her life. Jess appears for the last time in the season 6 episode "The Real Paul Anka" when Rory comes to the Open House of the new book store/art gallery/publishing house that Jess works at. He and Rory kiss, but nothing ends up happening because Rory tells him that she is in love with Logan. Jess did a lot of growing up and changing throughout the series, all for the better. Depsite his many faults in the earlier seasons, many fans believe that out of the three steady boyfriends Rory had throughout the series, he was the best match for her.