Is, from a certain viewpoint (Gaul's), Coriolanus Snow.
26 kids went into that arena - 24 from the districts, Coriolanus and Sejanus. Coriolanus was even ordered in there by the head gamemaker, just like all the others.
3 kids came out, and Coriolanus killed (or, at least, did his best to kill) the other two. Gaul greets him as "Victor" the next time they meet.
This is interesting, because it ties into another theory. I think Gaul would have let Coriolanus go, if he'd decided to leave with Lucy Gray. She would have considered him a failure and forgotten about him. At that point, she wasn't sure if his loyalties were sufficiently with the Capitol for her to accept him as her protege. But by betraying Sejanus and trying to become an officer, he'd passed her test and shown he was Capitol through and through.
Betraying Lucy Gray was just a bonus - Gaul probably didn't even know about that. Though she would have known that Snow was leaving 12 and heading for 2 of his own free will.