From the Super Mario Wiki
“I know speed when I see it, yes siree -- I'm the world champion sledder, you know.”
The Penguin Champ, also known as the Big Penguin, is a penguin appearing in Super Mario 64 and its remake, Super Mario 64 DS. He is a competitor character along with Koopa the Quick, and is only raced once in the game in Cool, Cool Mountain. He claims to be the world champion sledder, and seems to know Mario prior to the game's events, calling him "buddy".
The Penguin Champ is raced in the third mission of Cool, Cool Mountain. He can be found inside the house at the top of the mountain. When Mario drops in through the chimney, he will speak to Mario and challenge him to a race. If Mario accepts, he will have to race down the icy slide in the mountain and beat the penguin to the finish line. The penguin will give up a Power Star if beaten. However, if Mario takes a shortcut via skipping part of the slide by jumping onto another section or if he takes the hidden shortcut, the Penguin Champ will get mad and accuse him of cheating and he will disqualify him. When raced again after obtaining all 120 Power Stars (150 in the remake), he will become bigger and can knock Mario off the slide easily. If he is beaten, he will say that he decided not to slide for a while and let himself go.