Kenai was a young Inuit who is the youngest of three brothers, after Sitka, his tribe's chief, and Denahi, and is on the verge of becoming a man. But after rushing to get his totem from Tanana, the tribe's shamaness, which depicts a bear, an animal that symbolizes love (and ironically the one animal he hates believing that they are dangerous monsters), and forgetting to tie up his brothers' net so that the bears won't get their salmon, a bear attacks the tribe, and he and his brothers are forced to fight it off, but Sitka dies saving him and Denahi in the process. After getting into an argument with Denahi at Sitka's funeral, when he tells him Sitka sacrificed himself for them and does not blame the bear, resulting in Kenai throwing his bear totem in the cremation pyre where Sitka's remains (reduced to his shattered totem, broken spear, and tattered clothes, since the aftermath of a real fatal bear attack is too graphic to be shown in a kid's film) were cremated on, Kenai believes that the bear was responsible for Sitka's death, and decides to kill the animal at all costs, to avenge his brother. Kenai tries to convince Denahi that they should go after the bear, but Denahi refuses, claiming that he doesn't blame the bear for Sitka's death because Kenai provoked the bear's attack. Denahi advises Kenai against avenging Sitka's death, as it may anger the spirits, but a frustrated Kenai blows him off and sets out to kill the bear anyway.
Kenai succeeds in killing the bear, but for acting out of hate rather than out of love, as he had been commanded to do, the spirits turn Kenai himself into a bear as punishment for his actions and killing a bear who was Koda's mother and didn't kill Sitka, which would be revealed much later on. However, when Denahi arrives at the scene, since he had no idea that his brother was transformed into a bear, he falsely believes that the bear Kenai got transformed into "killed" Kenai and vowed revenge. As Denahi is about to kill Kenai, the bear is hit by lightning and is sent falling into a river, only to run into Tanana (who cannot understand bears' language) the next morning. Learning from Tanana, in order to become human again, Kenai had to travel to a mountain peak called, "the place where the (Northern) lights touch the earth." Along the way, however, Kenai meets an orphaned bear named Koda. At first, Kenai does not like the young bear's company, but eventually, he becomes attached to him. After escaping one of Denahi's (who still doesn't know that Kenai was turned into a bear) attempts to kill him by escaping inside a glacier, Koda tells Kenai that he was separated from his mother, and decides that Kenai should help him find her.
Kenai and Koda then befriend a pair of moose named Rutt and Tuke, and travel toward the mountains, but decide to ride on several woolly mammoths to get there faster. After getting off their mammoth, but leaving the moose behind, the two bears get into an argument ending with Koda abandoning Kenai and storming off alone, only to enter an abandoned village full of cave paintings showing bears being attacked by "monsters with sticks" (according to Koda). After getting directions from a pair of rams, Kenai and Koda try to work their way through a lava field while attempting to escape Denahi again, until the two bears arrive at a waterfall at the base of the mountain, where an entire troupe of bears are participating in a salmon run. After getting to know those bears more, Kenai realizes that he was wrong about them being monsters, and bears are really nice animals the whole time. The bear troupe welcome Kenai, and treat him as his new family, with Koda being his new younger brother, until it is time for them to tell stories to each other by passing a dead salmon to one another, and it is Koda's turn to tell a story, the one about how several hunters separated him from his mother.
While listening to Koda's story, Kenai realizes that the bear he killed in revenge for Sitka's death was actually Koda's mother. When Koda finds out, he runs away crying for losing his mother, and before Kenai could apologize, he gives up and heads up the mountain alone to find Sitka, only to find Denahi, now furious, waiting for him. As Denahi was about to deliver the final blow on Kenai, Koda, after listening to the mooses' advice during another encounter with them, saves Kenai from death by taking Denahi's spear to distract Kenai so that he can run off and get Sitka to change him back, only to accidentally drop the spear, allowing Denahi to try and kill him instead. When Kenai finds out that Koda is now the one in danger, he jumps between the two, attempting to sacrifice himself the same way as Koda's mother did before he killed her, and as a result, Sitka (whose ghost resembles an eagle, who was watching them the whole time) declares Kenai's bravery and act of brotherhood, and saves Kenai by turning him into a human again.
However, since Koda no longer has his mother (whose ghost arrives so that her son can say goodbye to her) with him, Kenai chooses to remain a bear permanently and stay in the woods and live with Koda, who needed him more than any of his own people. For this act of love, Kenai was declared to have become a man (despite the fact that he was restored to his bear form).
In Brother Bear 2, Kenai awakens from his first hibernation to find that spring has arrived (Kenai separated from Denahi, Tanana and his villagers because they do not need him). Even with snow on the ground and the trees still bare, love is decidedly in the air. Kenai and Koda scoff at the notion of romance, but Tug (the large, dark brown bear from the first film) cautions them (Kenai in particular) that "You can't run from love. It has a way of tracking you down." Kenai laughs, but later has a dream about Nita, a girl he used to know when he was young and a human.
Meanwhile, in her own village, Nita is preparing for her wedding to a man named Atka. But when the big moment arrives, the ceremony was interrupted by the great spirits. The earthquake destroyed everything in its path, it is revealed that Nita cannot marry Atka. The reason for this is that when they were children, Kenai gave Nita an amulet as a gift. What neither Kenai nor Nita realized was that the amulet bonded them as one. The only way for Nita to marry Atka is to go with Kenai to Hokani Falls, where he first gave her the amulet, and burn it.
Nita finds Kenai and manages to convince him to help her. But during the course of their three-day journey, their old friendship sprouts anew and flourishes. When Nita asks Kenai if he misses being human, he says yes. This causes Koda to run away up a mountain, thinking Kenai will leave him. A worried Kenai and Nita look for Koda as a snowstorm hits. They initially have no luck and Kenai keeps going, not noticing that Nita is stopping. Nita finds Koda, but an avalanche starts and as Nita screams, Kenai turns around and sees the snow heading right for them. The two are pushed down the mountain and soon get buried by the raging snow. Kenai rushes to save them and tells Koda he'll never leave him. Despite the fact that both are developing feelings for one another, Kenai and Nita burn the amulet and Nita returns to her village, only to find that where the amulet kept her from marrying Atka before, now her heart will not let her marry him either.
When Kenai wakes up the next morning, his moose friends inform him that Koda has gone to Nita's village to get her back. Kenai races to the village, knowing Koda will be killed. Back at the village, the villagers spot Koda and chase him up a tree. Then, Kenai races over to the rescue, grabs Koda, and flees. Nita hears the commotion outside and runs out of her tent. She is horrified when Atka goes after Kenai and Koda. She then chases after them to try to stop Atka. Atka chased Kenai into the mountains. Kenai hides Koda and runs off to distract the hunters. Atka finds Kenai and corners him on a cliff. They fight and Kenai is about to kill Atka when Nita runs over and yells at him to stop, but it's too late. Kenai lets Atka go and while he is distracted, Atka throws some embers in his face and kicks him off the cliff.
Atka wins and Nita cries out in dismay (because Kenai lost), pushes Atka out of the way and climbs down the rocks to help Kenai. Kenai is alive, though he failed to defeat Atka, and tells Nita he loves her and she tells him the same. The spirits come down and Nita is able to understand the bears again. Koda tells Kenai that he asked the spirits to change him back into a man so he could be happy. Kenai tells Nita that he cannot -- as he cannot leave Koda, but she says she can. So, Nita is turned into a bear that has the same coloring as Kenai, and they get married.
At California Adventure, Kenai appeared as part of the Magic of Brother Bear theme at Redwood Creek Challenge Trail, appearing in a totem ceremony show, a wood-carved sculpture at the entrance to the area and the namesake of Kenai's Spirit Cave, where guests are able to find their animal totem by placing their hand on glowing pawprints. In the summer of 2011, the Brother Bear theming would be removed and replaced with an Up theme inspired by the Wilderness Explorers. Though Kenai and Koda no longer appear in the area, the Spirit Cave was mostly untouched with the exception of signs removing Kenai's name.
- The scene where Kenai tells Rutt and Tuke "I'm not a beaver, I'm a bea-I mean, I'm not a bear, I'm a man!" after they call him a beaver after seeing him being turned into a bear by Sitka's ghost could be a reference to a scene in The Sword in the Stone where Arthur (Wart) and Merlin are turned into squirrels by Merlin's magic, and Merlin tells the Old Lady Squirrel, "I'm not a boy, I'm a squir-I mean, I'm not a squirrel, I'm a man!" after she falls in love with his squirrel form.
- Since Kenai chose to remain a bear permanently at the end of the first film, the only time we ever get to see his human form again in the sequel was him as a little boy in a flashback near the start of the film, when he meets Nita for the very first time.