Hotaru Kumogakure

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 mouthtomouth some of her familys secret medicine thus allowing him to defeat her brother. Afterwards she moves into the neighborhood and starts attending the same school. Taken from Wikipedia