Kirby

Kirby is a young Star Warrior. He is spoken of in legend as Kirby of the Stars. Kirby himself is implied to be a creation of Nightmare, but he was considered a failure. Kirby is like any other child, though with a much larger appetite. He loves to play and sleep, and can be utterly fascinated by even the most mundane things. He has a strong spirit, and loves to help people—sometimes even his enemies. He even demonstrates a maternal side on occasion. Since Kirby is only a baby he does not speak much, only saying "poyo". Kirby often shows that he understands language—he simply doesn't have the motivation to reproduce it himself, though he has the ability.

The official explanation of why Kirby doesn't speak was that his creator Masahiro Sakurai did not want him to.