Shellbeard

Shellbeard is the main antagonist in the spin off Club Penguin novel "Stowaway! Adventures at sea" as well as a major antagonist on Club Penguin Island (the sequel/spin off to the original MMO). He is a humanoid pirate turtle and the arch nemesis of Captain Rockhopper.

Stowaway! Adventures at sea.
The reader and Rockhopper discover Shellbeard on an island, where he appears to be marooned. The reader and Rockhopper take him aboard the Migrator and Rockhopper asks the reader to give Shellbeard a ship tour. He then reveals his true nature and steals Rockhopper's maps.

Club Penguin Island
The player first encounters Shellbeard after Rockhopper finds and shows the player Shellbeard's hat who apparently died after disappearing whilst searching for treasures around the islands surrounding Club Penguin Island. Shellbeard's ghost appears from his hat and explains some of his travels to Rockhopper and the player before being cut off by something for unknown reasons. He next appeared after he was freed from an enchanted gem in favour of Rockhopper being kidnapped, he then challenged the player to a duel. During the battle he summons several monsters to distract the player, he later chases the player down a zip line before getting a snowball launched at him sending him into the enchanted gem which sucked him inside. The gem then fell back into the water.