Kirby

カービィ

Kirby is a young Star Warrior. Legend refers to him as Kirby of the Stars. However, Kirby is actually a creation of Nightmare—though he was deemed a failure. It remains unknown what occurred afterward, or how Kirby came to be in the starship that crashes onto Pop Star. Since a Star Warrior's vessel is designed to travel wherever monsters are present, Kirby's ship detected the creatures Dedede was commanding, causing him to awaken 200 years ahead of schedule. As a result, he is still merely a baby.
Despite this, he eagerly fights for his friends. In battle, he displays remarkable skill, although he occasionally requires a little assistance.
Otherwise, Kirby resembles any other child, albeit with a significantly larger appetite. He enjoys playing and sleeping, and can become completely captivated by even the most ordinary things. He possesses a strong spirit and loves helping people—sometimes even his enemies. He also occasionally exhibits a maternal side.
Because Kirby is only a baby, he does not speak much, uttering only "poyo." Certain characters, such as Kine and Meta Knight, have acted as if they understand him, and Kirby uses this sound quite expressively. Everyone else must rely on his tone and body language. Occasionally, he speaks their language, with his favorite word being "suika" (Japanese for watermelon) or by repeating snippets people have said. He also commonly announces his own copy abilities or shouts out his attack names. (Of course, much of what he says is removed entirely in the English dub.) Like Pikachu from Pokémon, Kirby also retains his original Japanese voice for the dubbed version, and is notable for being the only character who does so.
Kirby often demonstrates that he understands language—he simply lacks the motivation to reproduce it himself, though he has the ability. Meta Knight stated that Kirby should have slept in his starship for 200 years, and because he was "born" too early, he lacks knowledge and training.
The official explanation for why Kirby does not speak is that his creator, Masahiro Sakurai, did not want him to. Characters who do not speak are often created that way to be perceived as more endearing and easier to relate to.