The Whale Cellmate is mainly a large, gray whale. He also has a scar on his face. Along with the Purple Cellmate, the Whale Cellmate 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 chum makes great for pulling off crimes. Eventually, the Whale Cellmate was used as a battering ram to break into the Krusty Krab and he pulled open the door to the safe that was 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.
Some fans speculate that he may be Pearl's biological father, given his strong physical resemblance to her. This could explain why she and Mr. Krabs are different species.
In the episode One Coarse Meal, it was revealed Plankton was scared of whales, but Plankton does not fear the whale cellmate.