This character suffers from Chuunibyou.
Chuunibyou or "Middle-school 2nd Year Syndrome", is a colloquial and rather derisive term in Japan which describes a person at the age of fourteen would either act like a know-it-all adult, or thinks they have special powers no one else has. Some would even go as far as being obnoxious, arrogant, and often look down on adults or older people. This way of thinking or acting is mostly seen in teenagers during adolescence, however there are people who still act like this even after reaching adulthood.
Do not use this tag for cases where this way of thinking or acting is natural for the setting (e.g. for a mahou shoujo heroine in a fantasy world with actual magic powers) or when the character REALLY wields supernatural powers. Only use it where this behaviour is out of place.