The Law of Ueki Plus

Two years after the tournament of the next god, Kosuke Ueki is now a 3rd year student in Hinokuni High School. For some reason, everyone in the real world has lost their memory of their most important person, including his friends, who participated in the tournament two years ago. Ueki, being the only person left in the world unaffected, is fighting in order to restore everyone's memories. Ueki follows a little sheep named U-lu to another world called Hankagai. There, he meets new companions and gains new powers.

(Source: Wikipedia)