Who's This? No, not the Coca-Cola mascot distilled from legends both Pagan and Biblical (or stolen from White Rock Beverages' 1915 publicity campaign, whatever). I mean the character that's been traipsing around the DC Universe for decades, but who nevertheless didn't get an entry in Who's Who. 'Tis the season to boo!
How you could have heard of them: You've been alive at some point after the 1930s, right? So you recognize the red suit?
Example story: DC Comics Presents #67 (1984) "Twas the Fright Before Christmas!" by Len Wein, E. Nelson Bridwell, Curt Swan and Murphy Anderson
So IS the idea of Santa Claus any more absurd than a Superman from another world? The DC Universe has magic, and super-science, and the gods of many pantheons, and powers that can't really be explained, so why NOT Santa Claus? Think about it. It's also got Uncle Sam made flesh. If the spirit of America can be a superhero, the spirit of Christmas damn well could too. In other words: WHERE'S HIS WHO'S WHO ENTRY?!??? He's got his own logo and everything, DC!
Who's next? A less believable character, I promise.