Who's This? The top half of page 19 in Who's Who vol.XI.
How you could have heard of him: The Crisis reboot made him a former cop rather than a Vietnam vet, and he more than dated Barbara Gordon, he was engaged to her. She broke the engagement after the events of the Killing Joke, and he never knew why or even that she'd been paralyzed until they met again in the pages of Birds of Prey, where he made occasional appearances after that. He's also appeared in issues of Robin , Red Robin, Batgirl Year One (where he was involved in her origin story), Batman, Detective Comics, Justice League: Cry Justice and even Firebrand. Told you he appeared plenty, right under my nose. A Jason Bard has appeared in Young Justice too, though he's still a soldier and not yet a P.I.
Example story: Detective Comics #425 (1972)
Who else? Though it looks like I might've skipped Jason Bard, there's another half-pager a few pages later that surely deserves a turn. Stay tuned!