He is the brother of Kenai, the film's main protagonist. He has a normal brotherly relationship with Kenai and their eldest brother, Sitka, until Sitka is killed by a bear in a fight that Kenai provokes. After that, Kenai sets out for revenge, but Denahi blames Kenai rather than the bear. Kenai eventually kills the bear, but, as punishment, the spirits turn him into a bear. By the time Denahi arrives, Kenai's transformation is complete, leading Denahi to believe that the bear killed Kenai as well and swears revenge for the death of his two brothers. He continuously sets traps for and attacks the bear until the end of the film, when he discovers that the bear actually is Kenai, at which point he ceases and accepts his brother's fate. His role as an antagonist is an interesting one, in that he is actually fighting the one he thinks he is getting revenge for. The film is also narrated by his older self.
Denahi did not appear (physically or verbally) in Brother Bear 2 because his voice actor, Jason Raize, committed suicide in 2004, while the sequel was in pre-production, two years before it was released. Another reason for his absence may be due to the fact that Kenai had chosen to remain a bear permanently, and does not need him anymore. Though it is possible that they meet up with each other offscreen.