Comsome! ~Combination Somebody~

The protagonist Kyoutarou transfers to the 'Senshu School' to pick up the trail of his deceased aunt—a notorious troublemaker infamous for her anti-social and destructive deeds. His goal is to find his sister, who vanished mysteriously about ten years ago. One day, he spots a girl in trouble with a ruffian. He helps her escape and buys her a meal, only for her to steal his bag and bolt while he's in the restroom. Days later, she shows up at his doorstep claiming to be his sister. An obvious lie, but she carries his aunt's business card. He lets her stay, figuring it’s a sign.