I think Quintas was basically his executive officer (second in command), though I'm unsure of the actual Roman position he held. I'll look that up if I have time tonight. He didn't betray Maximus in the encampment but was following orders from the person he thought was emperor.
He later commanded the Praetorians in the arena and ordered them to resheath their swords when Commodus demanded a sword during his duel with Maximus.