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 to Mario, though nevertheless is considered a bully by the penguins and Toads. If Mario touches Guppy, he will bounce back on the ground but he will still not take damage. There is a glitch if you are in the water touching Guppy manny times you will lose life. He appears in the Sea Slide Galaxy and the Deep Dark Galaxy.
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.
