Colin Milkovich is the brother of Mandy and Mickey Milkovich.
He appears in season two when Frank walks in on Ian and Mickey having sex at the Kash and Grab. Mickey is worried that Frank will tell someone what he saw so he asks Colin and Iggy to help him kill Frank.
Colin is shown as having the typical Milkovich loose moral compass; when Mickey says the person he needs help killing isn't someone they care about, Colin simply asks "Knife, gun, or tire iron?" Later, when Mickey lies, saying that they are killing Frank because he raped a girl, Colin asks casually "Statutory or catch and release?"