Who's This? He's the space beaver who (controversially) rated half of page 16 in Who's Who Update '87 #2.
How you could have heard of him: If you missed those issues, and Who's Who's '87 Update, it's unlikely. His stories were reprinted in the Tales of the Green Lantern Corps collection Vol.3 in 2010, in the middle of Lantern-mania, however.
Example story: Green Lantern Corps #203 (August 1986) "The Diabolical Doctor Ub'x!" by Steve Englehart, Joe Staton and Mark Farmer
Alright. I'm back. I'm okay. You okay? Okay, good. We're all okay. Except Ub'x who hasn't been seen since those appearances. Not even in an Ambush Bug or a Bizarro or a Multiversity comic. Come on DC. Now that you have Canadian superheroes, it's time to put this beaver back on the map.
Who's Next? Well, there's a Golden Age character who didn't show up in the first volume of Who's Who yet was mentioned a heck of a lot. Maybe go back and see what he was all about.