Why the Costume, the Name, the Secret Identity
The personal dimension of heroism, I can say now, is the essential reason that a superhero keeps his or her civilian identity a secret, whether he or she hides it or not. On the one hand, the civilian identity is irrelevant during dire circumstances. On the other hand, the heroic identity is irrelevant during circumstances that are not dire.
The secret identity, of course, may be explained in some other way within a story or in some additional way outside of it. Some other reasons are: (1) to protect the superhero’s loved ones from enemies; (2) to preserve loved ones from excessive worry for the superhero; (3) to permit a superhero to recuperate from battles without withdrawing from human interaction; (4) to give a superhero access to assets that will be useful to their superheroics, like places, knowledge, money, or equipment; (5) to allow a superhero to live a normal life, which a person might also want to do (see Brad Bird’s The Incredibles); (6) to be completely armored against attacks; or (7) to offer life support in hazardous conditions. Those are mostly in-story reasons.
Creators may hide the superhero’s identity behind a mask for more reasons: (8) to conform to the conventions of the genre, without thinking more about them; (9) to conform to them in order to win over to the readership; (10) to conform in order to comment on the things that established those same conventions; or (11) just to make the character look such and such a way.
Maybe there are more reasons, but the essential reason, I say, is the most essential distinction of a superhero, the commitment, because of a certain polarization.
In “ordinary” circumstances, most facts about a person are more or less relevant, which is to say, more or less irrelevant. They are irrelevant to the circumstances in which a person finds himself or herself and irrelevant to forming that person’s identity, because “ordinary” people are so very much like one another. In dire circumstances, the relevance of almost every fact about a person reduces toward zero, even those that form a person’s civilian identity, while the direness of the need, the need for salvation, escalates toward infinity. What decreases in relevance may be the person’s hometown, hair color, marital status, or favorite song, or again, the person’s annual income, level of education, preference regarding pets, or driver’s license number.
What increases the most in relevance is whether the person makes a commitment to act, supposing that that action will bring salvation. Everything else increases in relevance insofar as it puts the person at an advantage or a disadvantage for the action. This polarization makes it very reasonable for a person to wear a distinctive costume, one that de-accentuates most of what belongs to the person’s civilian identity and accentuates his or her heroic action. The costume may have other in-story purposes, of course, to increase agility, to inspire hope or fear, to represent an organization, or any of the other purposes that I named above. A costume may have out-of-story purposes, too, to appeal to the reader or viewer on any number of bases, for example, bright color, dark color, or looking good in action, or sex appeal, or again, to display an artist’s distinctive style.