An alter ego (=the opposite side of the personality, counterpart) is a second self, a second personality or persona (=an individual's social facade, the personality that a person projects in public, image) within a person. It was coined (=invented) in the early nineteenth century when schizophrenia (=a serious mental illness in which someone's thoughts and feelings are not based on what is really happening around them) was first described by early psychologists.

The term alter ego is commonly used in literature analysis and comparison to describe characters who are psychologically identical, or sometimes to describe a character as an alter ego of the author, a fictional character whose behavior, speech or thoughts intentionally represent those of the author.

The characters Doctor Jekyll and Mister Hyde in Robert Louis Stevenson's thriller Strange Case of Dr Jekyll and Mr Hyde represent an exploration of the concept that good and evil exist within one person, constantly at war. Edward Hyde literally represents the doctor's other self, a psychopathic being (=a living thing, especially a person) unrestrained by the conventions of civilized society, who shares a body with the doctor.

The term and concept also frequently appear in popular fiction, such as in comic books, for the secret identity of a superhero, vigilante (=someone who illegally punishes criminals and tries to prevent crime, usually because they think the police are not doing this effectively), crime fighter or villain. Related concepts include avatar, doppelganger (=a ghostly counterpart of a living person), impersonator (=to assume or act the character of, personate), and split personality.

I couldn't understand the bold part. Please help.
A character takes on/adopts another name, personality and even aspect in order to fight crime.
Superheroes, villains and other similar characters in popular fiction like comic books often have alter egos.
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Will this re-write help: 
The term alter ego and the concept of alter ego also frequently appear in poplar fiction, such as in comic books, and is used to denote the secret identity of a superhero, vigilante, crime fighter or villain.
Could you specify your problem more definitely?
Question to the experts: Is the "in" typeset in boldface necessary?
EDIT:Ooops, looks like the BOLD button doesn't work for me... I meant the second "in" in the sentence. 
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eg. Superman is Clark Kent's 'alter ego' and Spiderman is Peter Parker's.