It's Too Sick to Call this Love, Koikimo

Amakusa Ryou, a high-spec businessman, and his high school sister's best friend, Arima Ichika, meet due to a chance encounter. Their interaction is marked by Amakusa's forward approach and Ichika's initial disgust, but he mistakenly interprets her insults as signs of affection. This unusual dynamic forms the basis of their relationship, which is central to the series.