The whale cellmate is a large, light blue sperm whale who has a scar on his face. Along with the purple cellmate, he appears to be the strongest of the group.
The whale cellmate first appears outside discussing over the escape plan with Plankton and the other inmates. He reveals that they could escape prison with chum since that food makes great for pulling off crimes. Eventually, the whale cellmate is used as a battering ram to break into the Krusty Krab and he pulls open the door to the safe that is holding the "formula."
Unfortunately for him, Plankton, and the other cellmates, the entire Bikini Bottom Police Force was hiding in the safe and they were all taken back to prison.
In "One Coarse Meal," it is revealed Plankton is afraid of whales, but he does not fear the whale cellmate.