Book 11 of the Seishun Buta Yarou series.

A miniskirt-wearing Santa appears before Sakuta, calling herself Touko Kirishima and cheerfully informing him that she’s been handing out presents—Adolescence Syndrome. Apparently, one of the lucky winners is Sakuta’s classmate from middle school, Ikumi Akagi. According to urban legend, posts tagged #dreaming are filled with prophetic visions, and Ikumi is using those posts to play hero. Sakuta sees the results of this first hand when he spots her saving someone. And then a poltergeist suddenly attacks, and this is somehow his fault?! Solving all this might require delving into Sakuta’s unpleasant past...

(Source: Yen Press)