Chuuni

Chuuni is an internet slang term used to describe someone who acts or portrays themselves in an exaggerated, dramatic, or melodramatic manner. It is derived from the Japanese word “chuunibyou,” which translates to “middle school second-year syndrome,” referring to the tendency of some teenagers to develop delusions of grandeur and fantasize about possessing special powers or being part of a secret organization.

Examples:

When he found out he could move objects with his mind, he started wearing a cape and telling everyone he was a superhero. What a chuuni!
She always talks in a deep, intimidating voice and struts around like she’s the queen of the world. Such a chuuni thing to do!