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, oftentimes leading to the destruction of the hero's alter-ego and/or the villain using a loved one(s) as bait. In some cases a Super-Hero chooses to keep their identity secret to avoid any legal ramifications (ex. Lawsuits) or public scrutiny that would ruin their life.