Hotaru is a ninja. She confesses her love to Mamoru when they first meet. She lost to Yamame in a ninja athletic meet after being distracted by Yamame pretending to see a UFO. She and her brother attack Mamoru in order to avenge their disgrace. Her feelings for Mamoru prevent her from defeating him. She saves Mamoru by giving him (via mouth-to-mouth) some of her family's secret medicine thus allowing him to defeat her brother. Afterwards, she moves into the neighborhood and starts attending the same school.

(Taken from Wikipedia)