こんそめ！ ～combination somebody～

The protagonist Kyoutarou transferred to "Senshu school" to catch up with his deceased aunt - a troublemaker, infamous for her anti-social and destructive activities. Kyoutarou's objective is to find his sister, who mysteriously disappeared about ten years ago.

One day, he finds a girl that's caught in trouble with some ruffian. He helps her escape and treats her a meal, but in exchange she steals his bag and runs away while he is in the washroom.

A few days later, she came to his house and said she was his sister. Obvious lie, but she had his aunt's business card. He allowed her to live in his house, believing that it was sort of a sign.