Nightwing like Harley, is as successful as a former supporting character can get (they only cancelled his book to relaunch in under another title) but of course
he's also a Batfamily member.
I mean should we consider Deathstoke an A-Lister or not?
He's in plenty of videogames, was the main baddie of the Teen Titans toon.
But he's another character that get a series every 5 years or so that lasts about 25 issues tops.