Captain Del Fail was a UNSC captain in command of the UNSC Infinity, a mobile city designed to be the most advanced ship in the UNSC navy. He was angry all the time, and often shouted at his crew, who never do what he says.
He meets Johnny and the malfunctioning Cortana on Requiem, and shouts at them too. A swift metaphorical kick up the backside from Johnny showed Del Fail who was boss, but he still raged at Johnny and told him to surrender Cortana for decommissioning. Johnny said no.
In a rage, Del Fail abandoned Johnny on Requiem and returned to Earth, but Fleetcom didn't like the fact that Del Fail had left Johnny on Requiem, and confiscated the Infinity from him, putting it in command of his awesome first officer.
Del Fail lived out the rest of his days commanding the worst ship in the UNSC fleet, the UNSC Mouldy Terrantime, only to be blown up by Cairo Station's defences because Lord Hood found the name offensive.