Leo was a Human bartender employed at the 602 Club during the 2140s.
He tended bar the night Jonathan Archer received the news that his friend A.G. Robinson had been chosen over him to fly NX-Alpha. Archer offered to buy Robinson a drink, but Robinson instead bought the drinks. (ENT: "First Flight")
Leo was played by background actor Lemuel Perry, who received no credit for this appearance.
Leo's name wasn't scripted; the final draft script of "First Flight" referred to a "bartender" who was commonly referred to in the script as exactly that, establishing no name for him.