Koikimo

恋と呼ぶには気持ち悪い

Ryou Amakusa is a successful, easygoing businessman who treats dating casually, until a chance meeting with Ichika Arima, his younger sister Rio's best friend, turns into an obsession. Ryou pursues her with blunt, over-the-top gestures, and Ichika responds with open disgust and insults, which he reads as her own awkward way of flirting. Rio ends up stuck in the middle, trying to referee a mismatched pairing between a grown man and a high schooler.