Captain Marlin is a penguin who captains the submarineRed Raktuber in the quest Hunt for Red Raktuber. During the quest, the player discovers that a sea slug has taken control of him, as the Captain is seen with a sea slug "hat". However, in a Game Update FAQ, Mod Osborne stated that he liked the idea of Marlin just believing that he was under Mallum's control, but that he may not. This is why he appears to be mad, and is obsessed with the sea. After Pescaling Pax's devious plan to both retrieve the submarine (which had rebelled against Pax) and capture Larry succeeded, Marlin was imprisoned by the KGP for being "totally insane".
His name is likely to be a reference to the late Marlon Brando's role in the film Apocalypse Now. Another tribute to the actor is his examine, which states "The Don of penguins", referencing Brando's role in The Godfather films. Finally, the statements Marlin makes regarding all penguins being slugs is yet another reference to a Brando film, The Island of Doctor Moreau.