Roll20 is system-agnostic. The vast, vast majority of games can be played just fine with Base accounts; the features of Supporter and Mentor accounts are nice, but not required to play. (There are a small number of systems with complicated dice mechanics that would require the API to function 100% correct, but it's uncommon. Some people have created ports of boardgames or cardgames which require the API, but the main target is RPGs.) The list of games you can put in your profile and on the campaign LFG is based on what systems are actually being played frequently; the Roll20 team doesn't want people searching for a system to find 0 games, so the intent is that only games which have LFG results 24/7 get listed, with everything else filed under "Other Games." The list of character sheets is entirely dependent on users like you and me creating the character sheet. I haven't played a game using the HERO system, but I doubt it's got any problems in its system that are insurmountable for playing it on Roll20.