Guppy

"What, you again? You got to be kidding me! You want to run me out of this lake? You got to get through all eight rings!"

- Guppy

Guppy is a minor character in Super Mario Galaxy. He is an orca who will appear in a few of the game's missions. In two of the missions he'll "take over" a large body of water, and if Mario wishes to take it back, he'll have to swim through eight rings in a set amount of time. After Mario does that, Guppy will reward him with a Power Star.

Guppy is a harmless creature and contrary to his menacing look, he won't ever attack Mario, though is considered a bully by the penguins and Toads nevertheless. If Mario touches Guppy, he will bounce back on the ground, but he won't take damage. He appears in the Sea Slide Galaxy and the Deep Dark Galaxy. After being beaten in Sea Slide Galaxy, he becomes more friendly and actively helps the Toads look for the Silver Stars and Purple Coins during those missions.

According to the Prima guide for Super Mario Galaxy, Guppy was originally going to be a dolphin, though the developers later decided to change it.

Trivia

 * If Mario or Luigi touches Guppy's tail fin, he would be hit as if he had touched an enemy, but does not lose health.