Archer isn't affiliated with any groups, but he's definitely restless from just running and hiding from the plague doctors, so he'll be more willing to cooperate. He's ... an Archer-class Servant, so he fights primarily with projectiles -- namely, firing multiple weapons from portals. He's also armed with a real bow and arrows, and a sword.

He's versatile, so he'll go wherever as long as he gets to see some action.