Guardian is there because, unlike Thor, Beta Ray Bill is far more reactive than active. Keeps his head down, generally. He definitely qualifies as a Champion, though, if we want to go down that road.
...I think it being Champions and Guards is worse than Champions and Guardians, though. Guard sounds pretty wimpy to me.
Also, he's technically a Cyborg construct physically, not a Korbinite. Brain of a Korbinite in a bio-mechanical body. I dunno what we want to do there.