Fossa is the commander of the 15th division of the Whitebeard Pirates. His name was revealed in a SBS of Volume 58.
Fossa is a large and muscular man with an older appearance. He has a bald patch on the top of his head, a mustache, and a scar running diagonally across his face. He is often seen smoking a cigar. Fossa has Whitebeard's symbol tattooed on his right biceps, along with three small circles. He wears a sleeveless jacket and loose pants tucked into his boots.
His weapon of choice is a katana with a square tsuba, which he carries on his left hip, attached to his belt. Alternatively, he has been seen carrying it over his left shoulder with a long strap.
(Source: One Piece Wiki)
