Who's This? The stage magician at the bottom of page 14 of Who's Who vol. XVI.
How you could have heard of him: Mysto has no appeared since his little half-page in Who's Who.
Example story: "The Three Feats of Peril!" from Detective Comics #209, reprinted in The Best of DC #30 (1982), by Bill Woolfolk and Leonard Starr
Who else? I thought of the Monocle, who really did seem obscure when Who's Who came out, but he's since come into his own. Multi-Man would have made it back in the day too, but the comedy Injustice League saved him from this fate. Next up is a character that did eventually get a lot of play, but was really obscure at the time (still is, really), so I'm nothing if not inconsistent.