Who's This? A minor underwater(-ish) hero from the Golden Age, who nevertheless got a full entry on page 22 of Who's Who vol.16.
How you could have heard of him: As a Young All-Star, his origins were retconned to make him the mutant son of the real-life Captain Nemo who had forced aliens to make him a high-tech submarine. The man eventually changed his name to Perkins and married a Titanic survivor. I guess Neptune literally had the sea in his genes. Neptune Perkins would actually appear a fair number of times in the contemporary DC Universe, first in some issues of Peter David's Aquaman as the father of a new character called Deep Blue, and then having become a U.S. senator, a member of the committee monitoring Young Justice. He was killed in action in Infinity Crisis #3. So yes, he appeared quite a bit, but as with most Golden Age heroes, I'm really interested in what they were like BACK THEN.
Example story: "The Dweller in the Sea" by Gardner Fox and Joe Kubert, from Flash Comics #66 (1945)
Who else? Between Mysto and Neptune Perkins, the only other possibility might have been Nemesis, someone I'd never heard of at the time, if only he hadn't soon become much more prominent thanks to the Suicide Squad, and later as Wonder Woman's boyfriend. Up next, one last hero from volume 16?