Kazooie's personality is very different from Banjo's. She is a wisecracking, hot-tempered, slightly egotistic, tell-it-like-it-is loudmouth. She rarely ever shuts up, and then only because whoever she was insulting has finished the conversation. This could be because almost everyone that Kazooie has met had insulted her in some way. Bottles, for example, called her a 'strange friend' and asked Banjo if she could talk, and Mumbo Jumbo called her the 'filthy feathered one'. This could also be why Banjo and Kazooie are such great friends since Banjo hardly ever insults anyone(and if he does he usually apologizes). Kazooie likes adventuring (something which Banjo hates) and saving the world, while she hates boredom, Banjo's lack of an attitude, and facing backwards.
Aside from fitting into a backpack the size of her torso with no apparent discomfort, Kazooie can fly, carry Banjo, shoot eggs out of her mouth or firing them at enemies by laying them, and hatch large eggs when alone. In Banjo-Kazooie: Nuts and Bolts, she was given a magical wrench that not only allowed her to smack her enemies, but also let her magically carry objects and turn pipes.
According to Banjo-Kazooie: Grunty's Revenge and her Facebook page, Kazooie was originally discovered by Banjo on Breegull Beach inside the blue backpack one day. For whatever reason, the two became friends at some point after that, and Kazooie lived an annoyingly-quiet life with Banjo and his sister Tooty for an unknown period of time.
In 1998, Kazooie finally convinced Banjo to go out on an adventure somewhere, although Banjo insisted on taking Tooty along as well. Kazooie got her wish when Gruntilda captured Tooty, forcing a rescue operation. Kazooie throughly enjoyed learning cool moves and witch-bashing, although having to deal with Bottles, Mumbo, and Banjo's constant desire to help everyone was a constant pain in the butt. Eventually, Grunty was buried and Tooty saved.
A few months afterwards, Klungo devised a plan for Grunty to escape her rock prison and go back in time, preventing Banjo and Kazooie from ever meeting. Kazooie was taken back in time by Mecha-Grunty, and because of an odd glitch in the space-time continuum, this would prevent her past self from meeting up with Banjo. Luckily, Banjo followed Grunty with Mumbo's help and rescued Kazooie. The two of them proceeded to beat Grunty and restore the timeline.
Convinced that they hadn't seen the last of Grunty, Kazooie insisted that she and Banjo continue practicing their moves (and secretly hoping to get into the Smash Bros. cast). In 2000, her hunch proved correct when Grunty's sisters rescued her and blasted Banjo's House in he process, killing Bottles. Kazooie was instantly pumped to get on another adventure, and while Jamjars was every bit as annoying as his dead brother, she learned to Split-up from Banjo and lay the hurt on the witch's minions herself. After defeating Grunty a third time and reviving everyone who died, the duo went into "rest mode" once more.
Kazooie, however, wasn't pleased with doing nothing. She jabbered constantly at Banjo to keep practicing moves. But Banjo flatly refused this time, and decided to start fixing up his house...for a few weeks, anyways. After that, the pair slipped into a depressing lifestyle of boredom and pizza.
After eight whole years of nothing, Grunty reappeared in 2008. But another being also showed himself: Lord of Games, a near-omnipotent character that set up a vehicle-based challenge between BK and Grunty for the rights to Spiral Mountain. Kazooie was not one iota happy about this, since L.O.G. had swapped her moves for a magical wrench. After an arduous journey in which Kazooie constantly felt as if she got the shaft, Banjo finally defeated Grunty and legally earned the deeds to his home. Kazooie did get her old moves back, so it wasn't a total waste.
Nowadays, Kazooie is the same old foul-mouthed incessantly-blabbing bird as always. She still holds out hope of getting into the ring with "those Smash Bros. weirdoes", as well as a game not too far in the future that involves some more traditional witch-bashing.