Complex sentence: A secret identity is an element of fiction wherein a character adopts a pseudonym and dons a costume (ranging from a simple mask to a full-body outfit) while keeping their true identity hidden.
Split into simpler sentences: A secret identity is an element of fiction wherein a character develops a separate persona, usually adopting a pseudonym. The character often also dons a costume (ranging from makeup or a mask, to a complete outfit), while keeping their true identity hidden.