He dated the male protagonist's crush, Katie Collins, while in high school, much to the protagonist's disappointment. He is the Water Guardian if the protagonist (Ashley is the Water Guardian if the protagonist is female).
During the events of the game, the protagonist is able to change history so that Katie dumped Trent and instead she formed a relationship with the protagonist.
This is Trent's biography during the process of the story change.
Trent was in a difficult relationship with Katie, and he was madly in love with her. These difficulties were brought on because Trent was unable to get along with Katie's friends, causing tension between the two.
In the Blood path, Trent ends up in the Emergency Room. Katie sees him at the hospital, but he has no idea who she is.
In the Light path, Trent leaves Kate and dates another girl, Emma.