It's Too Sick to Call this Love, Koikimo

Once you fall for someone, you can't stop the love. A strange encounter spurs the meeting of Amakusa Ryou, a high spec businessman who is loose with women, and his high school sister's best friend, Arima Ichika. From there, he falls madly in love. On one hand, he approaches her with almost too straight-forward methods, while she responds simply disgusted, insulting him without hesitation... and he takes it as her way of showing love. This is a romantic comedy about a twisted elite employee and a normal otaku high school girl.