I like who I am and who my parents were. So I came up with the idea of a dual identity. [...] I think they need to see your eyes for that. So Clark Kent wears a mask instead of Superman. 
A secret identity is the idea that a person's civilian identity is unknown to the public at large.
History
Most heroes and even some super-villains utilize a secret identity for a variety of reasons. The most common reason is the fear that villains will seek to hurt a hero's loved ones in an attempt to draw a hero in combat or just to psychologically hurt the hero. Other motives include: avoiding the publicity that comes with being a hero, wanting to live a normal civilian life, and especially in the case of villains, stay under the radar with the police.
A secret identity can be maintained in a number of ways. One of the most practical, and common, of these is the use of a mask to hide one's features. Mask styles vary from a simple domino mask that covers only the eyes to a full face mask that obscures all the features. Superman reverses the concept; he wears no mask at all while in public. In his civilian identity of Clark Kent, however, he dons a pair of glasses and puts on a nerdy, klutzy act to convince anyone suspicious that Clark Kent couldn't possibly be Superman.
Even with a secret identity, those close to the hero may come to believe they are living a double life. To solve this, many heroes have had a partner or teammate dress as them so that both their civilian identity and hero persona can appear in public at the same time.
Though a secret identity may provide many benefits to those who chose to have one, there are those that find maintaining one to be either too much effort or unnecessary altogether. Elongated Man is a notable example of a hero who chose to divulge his identity to the public.
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