Sagako

So, uh, the protagonist is a Saga high school student living on an onion farm, who left the baseball club due to injury. He has a twin sister who was part of a team of women led by Tae (Sagako hunters) who exorcised Saga's demons called "sagako", and she was the only one with the power to seal them into onions. However, she used her power too much, which somehow caused her to turn into an onion. So the rest of the group kidnapped his twin brother in hopes that he had the same power, and he does, more or less. And they are going to train him to seal demons.

Demonios