Mahou Shoujo Twin☆kle

Main character gets invited to a rooftop after school. After a casual conversation Yayoi confesses that she is a magical girl. Of course protagonist takes it for a joke, but only for a minute till Yayoi repels a sudden attack of a monster. Yayoi is injured and needs to have the wound treated, so they come to protagonist's house. This day an unbelievable story of dating a magical girl begins.

[From vndbreview]