Frank moves with the easy menace of an ex-cop, because that's what he was. He is tough and incredibly loyal. He is the man Baba gets to change a plug or dispose of a body. For Frank, working for Baba is just a job, most of the time he sees the violence as unnecessary and is the steadying influence on Toby.
He is Toby's minder and mentor; the voice of wisdom that blind youth and psychotic tendencies won't listen to. Frank's menace stems from his efficiency and ability, yet he also displays qualities of benevolence that make him uncomfortably likable.