Characters whose alter ego is not known to the general populace.
Super-Heroes often choose to have a secret identity, (as do Super-Villains who often adopt aliases) when becoming a hero to protect themselves, their friends and their loved ones. The consequences of an enemy finding out their real-life identity could be most dire. Often leading to the destruction of the hero's alter-ego and or the villain using a hero's loved one as bait. In some cases a hero (or villain) chooses to keep their identity secret out of the possibility that they would face legal liability or public scrutiny which would ruin their life and make it harder to live if their secret were exposed to the public.