A girl is walking from one class to another. Some kids behind her playing around with their tennis rackets and a ball accidentally drop the ball and it bounces forward and passes her. As she watches the ball bounce forward, she sees a boy in front of her step on the ball, and suddenly, the ball is no more. It has gone.
After a lot of snooping around the girl realises that the boy is able to erase things from existence. A pretty terrifying ability right? He is only able to erase one thing per day. The one weakness of the ability is the power of memory; in normal cases, the tennis ball is erased so people even forget it existed. But since the girl witnessed the ball being erased, her memory of the tennis ball can bring it back.
The boy gets up to some evil shenanigans of course and the girl tries to stop him, but she tries to get her friend to always follow her and watch her in case she is erased. If there is even a single instance where her friend isn’t watching and the boy erases her (I guess he has to touch the object he is erasing in order for his ability to work), she is gone forever.