"Feature characters" - or "FCs" - are the characters we recognize from Heroes. They consist of both the main cast and those who are seen on a less-regular basis, perhaps as supporting cast.
Feature characters are expected to be active players. After three weeks of unreported inactivity, an FC may be re-opened for application. If a player resigned from an FC, that character will be re-opened for application.
FCs must go through chargen the same as original characters, but at the end of their application, they may be asked to "audition". An audition is just a quick and easy scene to make sure staff knows you have a handle on the character's personality and quirks.
On that vein of thought, they will be watched to make sure they're acting in character (we're not fanfic gone wild, folks). If staff feels that a particular FC isn't living up to their role by acting far out of character, staff reserves the right to intervene.
To see a list of currently available and unavailable feature characters, type +features (note: the +features command only works on the MUSH). If a character isn't on the list at all, that doesn't necessarily mean they can't be played; just talk to staff if you have a character in mind and don't see it listed.
To learn about the details of applying and going through chargen, see applying for a character.