He’s a Lazy Evil Spirit who feeds on negative aura, provided by students of Sankai High when they go jogging on a hill.
He's fat and rather monstrous.
As students of Sankai High ran up a very steep hill outside of school every year for the marathon competition, they grew exhausted and a negative aura increased, which attracted that evil spirit into haunting that hill. As more students come and expend energy running up the hill, he feats on their negative aura, growing fat and lazy.
Rinne tried to exorcise it, but because of the students not wanting to do the marathon, both he and then Tsubasa end up being affected by the evil spirit and laze around next to him. Using a Bouncy Soul Capsule provided by Rokumon, Sakura rolls it down the hill, crushing the demon, causing him and all the negative aura to be exorcised.