Ueki is a celestial who was sent to the human world when he was an infant. He was then adopted by a man and her daughter and started to live like a human being. Ueki goes to the Hinokuni High School with Mori.
Ueki is a boy who believes in justice due to an inscident when he was younger and was saved form falling from a building by Kobayashi, his teacher. Since then, Ueki idolizes Kobayashi and wants to become like him.
Ueki's power is the power to turn trash into trees and the condition to use his power is to squash trash in his hand.