Humans are a race of beings that exist in Pop Star's galaxy. They aren't anywhere near as plentiful as other races such as the Fairies or the Kirbys. As of now, only two humans (besides guest characters) are known to exist, both of which presumably still live peacefully in Pop Star.
It is open-ended if the two original 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 an original human character. Ado, a young artist, is fought at the end of Cloudy Park. She is targeted (and later possessed) by Dark Matter. As such, Kirby is forced to fight her. She 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 Nintendo games make cameo appearances in the game as well, but are not regular inhabitants of Dream Land.
In Kirby 64: The Crystal Shards, Kirby meets Adeleine, a young art student studying in the forests of Pop Star. She, like Ado, is possessed by a Dark Matter 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. Fans note its striking similarity to our Earth, as the seven continents and orbiting moon are visible. Additionally, the game's Japanese website mentions that Shiver Star's inhabitants had to leave the planet, leaving factories and empty buildings behind. It is possible that Shiver Star used to be Earth-like before its climate change.
Upon suggesting the anime, Kirby's creator, Masahiro Sakurai, established that Kirby: Right Back at Ya! would not involve humans. Even so, a few managed to slip into the episode Dedede's Monsterpiece as sculptures and paintings (all of which are parodies of classic artwork). Customer Service rents them to King Dedede, saying that they're from another planet -- it's implied that this planet is Earth. Tiff refers to the humans in the art as aliens.