One Day, out of the Blue, I Got a Gal's Forgiving Wife.

*"Hey, why are you always pretending to be asleep?"*

Miran Hanazuki, the most popular gal at school, often calls out to him, the bottom-of-the-barrel otaku, Shuji Nagasawa. He was confused by her flirtatious attitude, and then she unexpectedly declares that she's engaged to be his fiancee!? The beginning of a romantic comedy between two polar opposites, an introverted otaku and an extroverted gal!!

(Source: Kadokawa, translated)