Kirby is a legendary Star Warrior (with many baby or childlike qualities) who, according to legend, is supposed to save Pop Star from mass destruction, but is awoken by mistake 200 years early.
Because of his age he is still getting the hang of his powers and depends even more on the help of his friends to pull through tough situations.
It has also been inplied that Kirby was originally thought to be a creation of the ancient evil entity called Nightmare, but after refusing to follow his evil orders, Kirby was discarded into the depths of space.
He also has a terrible singing voice (not gonna lie. He does.)
Sonic is a blue anthropomorphic hedgehog who can run at supersonic speeds and can curl into ball- mostly to attack enemies.
And just like any other hedgehog, Sonic can not swim (which is why there are rarely any water levels in the games) and water happens to be his greatest fear, NOT Amy Rose.
Sonic also has the ability to use Chaos Control from the Chaos Emeralds and become "Super Sonic", which is a more faster and invulnerable version of himself.