Kazooie is a red-crested breegull, a fictional species of bird known for their fast running speed and flight capabilities. She is a brash, snarky loudmouth with a deadpan wit and no filter, traits which cause trouble more often than not. However, she is fiercely loyal to Banjo and willing to help with any situation that comes their way. Kazooie spends most of her time inside Banjo's backpack, residing in his house near the foot of Spiral Mountain.
In Banjo-Kazooie, Kazooie alerts Banjo that his sister Tooty is being kidnapped by the evil witch Gruntilda, who wants to steal her beauty, but Banjo wakes up too late to respond. Once he awakens, the duo set out to rescue her from Grunty's Lair, a complex cavern across Spiral Mountain with links to many disparate worlds. Along the way, Bottles the mole teaches Kazooie a variety of powerful abilities, including beak attacks, egg shooting, flight, invulnerability, and specially-designed footwear. After the pair get through the lair and confront Grunty at its peak, they ultimately summon the Jinjonator to push her off the tower to her doom; she crashes into the ground and is buried by the ensuing debris.
Two months later in Grunty's Revenge, Grunty's ogre-like minion Klungo creates a robotic body for her spirit to possess while her body remains underground. This newly-built "Mecha-Grunty" kidnaps Kazooie and travels 20 years into the past to stop the two from meeting, thus preventing her defeat. Kazooie is quickly rescued by Banjo, whom their friend Mumbo Jumbo has sent back in time to chase down Grunty. Via Bottles' grandfather Bozzeye, she re-learns some of the moves that she had forgotten due to the time displacement. The duo eventually send Grunty's spirit back to the future into her real body, restoring the timeline back to normal.
Two years later in Banjo-Tooie, Grunty's sisters appear to rescue the now-skeletal witch, who promptly attacks Banjo's house and kills Bottles. Banjo and Kazooie set out again to chase down the witches, who are attempting to restore Grunty's flesh by using their Big-O-Blaster to suck the life out of the entire Isle O' Hags. Bottles' brother Jamjars helps the duo learn to work independently, which is a huge boon to Kazooie; without Banjo weighing her down, she can move significantly faster and jump higher, while retaining almost all of her offensive abilities. Jamjars furthers her skills by teaching her first-person egg shooting, wing attacks, free gliding, and the ability to hatch eggs; however, she cannot swim underwater or grab ledges as Banjo can. Banjo and Kazooie triumph over Grunty once again, and use the B.O.B. to revive Bottles.
Eight years later in Nuts & Bolts, the duo has become lazy and out of shape from their lack of exercise, having forgotten their moves from the previous games. Grunty reappears as just a skull and attempts to start a fight, but before they can, they are interrupted by the Lord of Games (L.O.G.), supposedly the creator of all video games. He creates a robotic body for Grunty and a series of vehicular challenges for Banjo and Kazooie to go through. Kazooie continuously complains about being given the shaft in the game; her only use comes from a magical wrench Mumbo has given her, which allows her to carry objects and utilize simple attacks. The pair eventually battle and defeat Grunty, who is forced to work in L.O.G.'s video game factory, and earn back their old moves.
As playable characters
Main article: Banjo & Kazooie (SSBU)
Banjo and Kazooie were announced as playable fighters, alongside the Hero, during the E3 2019 Nintendo Direct on June 11, 2019. As always, Kazooie executes the majority of maneuvers while remaining mostly out of danger herself.
According to Masahiro Sakurai, Banjo & Kazooie were heavily requested characters for a long time even leading up to Ultimate's launch, particularly in the West. Even after character reveals like Ridley and King K. Rool, requests for Banjo & Kazooie still kept coming in. While their inclusion would have been a natural one in the past, they were sold to Microsoft soon after, and it was originally thought that Nintendo shouldn't collaborate with their console competitors. However, thanks to Rare and Microsoft's cooperation, the rights to the characters were easily secured, as Microsoft had developed a strong third-party relationship with Nintendo leading up to their Smash reveal.
In their gameplay showcase on September 4th, 2019, Sakurai openly acknowledges the fact that the intellectual property is completely owned by Microsoft. He also acknowledges Rare and Microsoft's cooperation in bringing the franchise into Super Smash Bros. Ultimate, as they were happy to see the duo in the game and saying Rare welcomed Nintendo with open arms. He even notes that if those interested in the series want to play the original titles today, they should do so in Rare Replay on the Xbox One.