Who's This? The disco god on page 25 of Who's Who vol.XXIV.
How you could have heard of him: Ultraa was reintroduced in Justice League Quarterly #13 (1993) as a native of the planet Almerac and Maxima's betrothed (poor guy, she was always over here trying to get another consort). He would later turn up as a League Buster in JLI vol.2 #65 (1994). Grant Morrison will be using him in The Multiversity (in Multiversity: Ultraa Comics).
Example story: Justice League of America #201 (1982) "A Hero for All Seasons" by Gerry Conway, Don Heck and Bob Smith
Who's Next? The character I consider possibly the most obscure of Who's Who's entire run.