Jamie Milkovich is the cousin of Mandy and Mickey and nephew of Terry Milkovich. He in season one when Ian rejects Mandy's sexual advances causing her to send her family members after him in revenge. Jamie is one of the three Milkovich's who try and hunt Ian down in this episode (the others being Mickey and Joey Milkovich).
In Can I Have a Mother, one of Mickey's brothers states that all the drugs were used up at a quincenera at Jamie's a week prior, implying that Jamie has, at this point, moved out of the Milkovich house.
Jamie is not shown again until season 10. In "Gallavich" Jamie is shown to be directing wedding guests to the Polish Doll. Terry Milkovich jumps on him and beats information out of him.