Jason Rosser is Eve's younger brother, and was a jail resident.
Jason was in human jail for stabbing a human girl, he was released when she dropped charges. After being released he began stalking Claire, and later stabbed Shane. It is also implied that he murdered several human girls starting just after his release.
He, along with Dean, attempted to drain Claire's blood and blame it on the vampires. Frank Collins intervened and saved her life.
Jason is an easily controlled child who is always breaking the law.
He successfully staked Amelie at the Feast of Fools, yet lived to brag about it later.
He was uneffected by his father's death, though he showed up to the funeral. He broke into the Glass house, and was chased down the street by Michael Glass, whom he shot in the chest numerous times.
He has a grudge against his sister Eve because she was the main reason he was in jail.
In one of her diary entries at the end of the early books Eve implies that he has used heroin, in Feast of Fools Claire states that he looks like he has used it.
In early books he shows a liking for becoming a vampire, he later achieves this.