English does not appear to be Mumbo Jumbo's first language; he always speaks in the third person and often doesn't refer to people by their name. For instance, he always refers to Banjo and Kazooie as "bear" and "bird" respectively.
In Banjo-Kazooie and Banjo-Kazooie: Grunty's Revenge, Mumbo transforms the duo into various forms. While his spells must stay within a limited distance of the originating world, the transformations make Banjo completely incognito and allow him to explore otherwise inaccessible areas.
In Banjo-Tooie, Mumbo no longer performs the transformations as this role is given over to Humba Wumba, but instead plays a more active role in the game. He is able to stand on a Mumbo Pad and, provided he has a Glowbo, can perform a spell designed to make things easier for Banjo and Kazooie. While on foot, Mumbo can't do much but jump, swim, and attack with his Zap Stick, and he has less health than Banjo. It is obvious he is extremely skilled in his magic, due to the fact that he is able to revive those who have died (as seen in Banjo-Tooie when he revived the Aliens) and even send a person back in time (as seen in Grunty's Revenge).
In Banjo-Kazooie: Nuts & Bolts, Mumbo builds all the odd vehicles that the duo dream up. He also plays roles in various other worlds.
Mumbo first meets Banjo and Kazooie in his hut at Mumbo's Mountain. He offers to use his magic to help the heroes as long as they have collected enough Mumbo Tokens to pay his fee. Aside from Mumbo's Mountain, Mumbo can also be found at Bubblegloop Swamp, Freezeezy Peak, Mad Monster Mansion, Click Clock Wood, and even hidden in Gruntilda's Lair. However, during the level Click Clock Wood, Mumbo will only use his magic during spring, because in summer he is too bothered by the heat to help them, in autumn he is very busy sweeping leaves out of his hut, and in winter he goes on vacation.
After Banjo and Kazooie saves Tooty from the clutches of the wicked witch Gruntilda, Mumbo joins the heroes for celebratory barbecue, claiming the bear and bird duo wouldn't have gotten so far without his transformation spells while cooking fish on a grill until Tooty told everyone that Grunty got away and still must be defeated, causing Banjo and Kazooie to make a mad dash to the lair for the ultimate showdown with the evil witch. Mumbo would later join his allies on a tropical beach after Gruntilda's defeat, where he will show Banjo and Kazooie pictures of secret items if they have collected all 100 Jiggies in the game and tells them to find out in Banjo Tooie.
In an alternate non-canon scenario during the game over sequence, Mumbo will appear with a flower for Grunty after she succeeds in stealing Tooty's beauty. This scene becomes unavailable for the remainder of the game after Grunty's Furnace Fun is completed as Tooty is saved and no longer in danger of having her youth taken from her, and Gruntilda is either waiting for Banjo and Kazooie at the top of her lair or stuck under a boulder if she is defeated.
Banjo-Kazooie: Grunty's Revenge
Two months after the events of Banjo-Kazooie, Mumbo discovered that Gruntilda had escaped imprisonment beneath a massive boulder in Spiral Mountain. He ran to tell Banjo of the trouble but arrived too late as Grunty kidnaps Kazooie and travels back in time to change history. Mumbo uses a spell to send Banjo after Grunty as well as send a message to Bozzeye to assist the bear.
Banjo eventually meets Mumbo's past self, who is in-training as a shaman and will transform Banjo into different forms if he collects Mumbo Tokens for him by defeating bosses. At the end of the game, Mumbo will return Banjo and Kazooie back to their own time period. However, depending the game files percentage, Mumbo will either goof up the spell and create a bunch of Banjo clones or he will go to his pad to learn a bee dance from Honey B.
Set two years after Banjo-Kazooie, Mumbo joins Banjo, Kazooie and Bottles in a game of cards at Banjo's house and had won most of the cash at the table. Suddenly, a brief but strong earthquake shakes the entire house, causing Mumbo to investigate outside. He discovers the Gruntilda had been freed by her sisters Mingella and Blobbelda who plan to restore her now-skeletal body. Mumbo runs to warn the others but is spotted by the witches before reaching Banjo's house. He succeeds in saving Banjo and Kazooie from Grunty's wrath, but Bottles is caught in a powerful explosion that destroys the bear's home. The next morning, after witnessing the death of Bottles, Mumbo tells Banjo and Kazooie to meet him at his skull so he can help them stop the witches.
Mumbo will appear in different levels and become a playable character if the heroes bring him a magical creature called a Glowbo. As Mumbo, the player can zap enemies with his Zap Stick and interact with Mumbo Pads to perform powerful spells. After Bottles is revived and Grunty is defeated at the end of the game, he will join Banjo, Kazooie, Jamjars, and Humba Wumba for a "kick-a-round" with Grunty's disembodied head, to the latter's chagrin.
Banjo-Kazooie: Nuts & Bolts
Eight years after Banjo-Tooie, Mumbo has become a mechanic in Showdown Town. He helps Banjo and Kazooie by building new vehicles they can use to complete various challenges.