The Bears are the main antagonists of the film.
He is very mischevious. Cub plays with Humphrey and told him that they're 'weird'. They continued playing, but then Humphrey got a bit rough and threw a hard snow ball at him/her and Cub started crying. Which later angered the mother(?) bear.
The Three Bears
Now running away, Kate and Humphrey get cornered on a cliffside, unable to go anywhere. The front bear decides to try and shake them off, but instead broke the tree loose and he and the two wolves tumbled down the cliff.
Still chasing, the bear slides his way and tries to get them, but fails and ends up crashing on the other side of the train.