Usamaro's true form takes on a shape of a small blue rabbit with a black mist for his tail. In his human form, he takes on a shape of a child with pale skin, light blond hair, and red eyes with blue and black pupils, with rabbit-like features such as ear-like hair buns and tail, and two blue teardrop-like whisker marks on each side of his cheeks.
Though a Demon, Usamaro does not appear malevolent overall. He has a childlike mentality, and enjoys making friends and having fun. Because of his naivety, Usamaro does not understand the importance of life's difficulties, thus he is compelled to use his powers to rid people of their painful memories and duties so that they may play with him.
Sometime in the past, a young boy found the starving Usamaro on the outskirts of his village. Feeding Usamaro, the pair became friends and played together and Usamaro moved into the boy's village. Usamaro, not understanding the nature of humans, Usamaro ate the villagers' bad memories, causing them to play and forget their responsibilities.
An exorcist, seeing what Usamaro had done, sealed him away, but it was too late, the village fell into ruin. Since then, a festival was to be held every eleven years, so that the event would not be forgotten.
Ao no Exorcist Movie
As shown in a picture book, Usamaro's past is relatively similar, however, much like Amaimon mentioned, the ending is different. Instead of simply being sealed away leaving the village to die out, Usamaro first saved the city. This was due to Rin's friendship with Usamaro and his teachings about the importance of memories, whether they were joyful or sad and painful. This can be seen thoroughly when Rin's memories about Shiro were nearly eaten by Usamaro. He told the Demon that although the memory had hurt his feelings a lot, he could never let himself forget about his 'old man'
Powers & Abilities
Usamaro is a demon that is related to the King of Time, Samael.
Memory Eating: Usamaro's primary ability is to eat memories. He primarily eats bad memories in an attempt to make others happy, though the notion is rather misguided. The memories appear to be stored in the mist that makes up his tail.
Time Eating: While used much more sparingly, Usamaro has the power to eat time, altering the course of events to change the present.
Transformation: Usamaro possesses the ability to swap between a human form and that of a small rabbit-like demon.