Kokoro Himeru no Zen Himitsu

Tsugumu Ichimonji, a world-renowned high-school-aged bodyguard, transfers to a Japanese high school to secretly guard Himeru Kokoro, the daughter of a wealthy tycoon. One day, he follows her to the rooftop, only to find her, surrounded by flames, facing off against a huge ◯◯◯◯ (it's a secret)! "Don't worry… I'm fireproof!" Her secret will open the door to an extraordinary world! (Source: Weekly Shonen Sunday, translated)