Se〇re ga Zenzen Warikitte Kurenai

Second-year high school introvert Hiryuu Akiyoshi has a friends-with-benefits arrangement with his classmate, Rio Wakana. Rio is an elegant, black-haired beauty born into a wealthy landowning family. Thanks to her overly strict parents, she's never really been allowed to enjoy normal teenage pastimes. To relieve that stress, she suggested they start a no-strings-attached relationship. Except... She wants to eat lunch together. Walk home together. Spend weekends together. She's doing everything except keeping it no-strings-attached. (Source: Kadokawa, translated)