Humans are a race of beings that exist through Pop Star's galaxy. They aren't anywhere near as plentiful as other races such as the Fairies or the Kirbies. As of now, only two humans (besides guest characters) are known to exist, both of which presumably still live peacefully in Pop Star.
There is much also debate whether or not the two only-known humans in the series are actually the same person, making it very possible that there is in fact just one known human across all of Pop Star's universe.
This was the first game in the series that featured humans. Ado, a young artist, is fought at the end of Cloudy Park. He is targeted (and later possessed) by Dark Matter. He is given no real backstory but is shown bonding with Kirby and the Animal Friends at the end of the game.
Several other human characters (including Samus Aran) from other games make cameo appearances in the game as well.
In Kirby 64, Kirby meets Adeleine, a young art student studying in the forests of Pop Star. She, like Ado, is possessed by one of Dark Matter's minions after she finds a Crystal Shard sitting by her canvas. She joins up with Kirby and Ribbon after she is freed from Dark Matter's ghastly presence.
Kirby is also able to travel to a frozen planet named Shiver Star in this game. Because it has its own moon orbiting around it, and because the seven continents are visible, it is often assumed by fans that this is (or was) Earth.