The Boy and the Beast (2015)

This is a complicated plot for an animated movie, which means that it was likely over looked when it came out in theaters in the United States.  The Japanese animated giants also have faith in their audience to handle the material, which makes for interesting watching.
Ren is the boy.  He is nine years old when his distant relatives try to take him into their custody. His mother has died in a sudden accident, and his father has not been contacted (he and Ren's mother divorced). Ren is so angry that he runs away, and is almost immediately abducted by Kumatetsu, a humanoid bear-monster who lives in Jutengai, a fantastic world ruled by animals who are human in all but their appearance. Kumatetsu is the hot head heir to Jutengai's throne, and he needs an apprentice in order to prove himself to Jutengai's enigmatic current ruler. So Ren is made to help Kumatetsu defeat his do-no-wrong rival Iozan.
Ren and Kumatetsu's do not get along and they do not complement each other. Ren helping Kumatetsu doesn't automatically confer life-changing or character-building wisdom to Ren. Instead, it gives him the courage to forge his own path. There is a happy ending to it all, but it allows the less-than-perfect viewer reflect on their own foibles.