The Boy and the Beast — Mamoru Hosoda

Tokyo's a big city-big enough that it's easy for a grieving boy like Ren to slip through the cracks. But Ren slips a little farther than he meant to, tumbling into the beast world of Jutengai! He ends up with a new name--Kyuta--and a new life learning the way of the sword as the apprentice of the bear Kumatetsu. Kumatetsu's got problems of his own, though, and the boy and the beast may have more to learn from each other than they realize. But the boy's arrival in the realm of the beasts has put both their worlds in danger, and they're going to need more than life lessons to face it!