Alvin is the great-uncle of Plankton. He only appears in the comic A Plankton Family Tree. Alvin's last name is not known, since he comes from Plankton's mothers side of the family. Plankton explains that Alvin has not been heard from for a long time and that all that was found of him was "his hat in a pool of ketchup."
Alvin is green like much of the rest of his family. He wears a pink top hat and looks like he may be a krill rather than a plankton.