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 older looking man, with a bald patch on the top of his head, a mustache, a scar running diagonally across his face, and seen smoking a cigar. He has Whitebeard’s symbol tattooed on his right biceps, along with three little circles, and he wears a sleeveless jacket, and loose pants tucked inside his boots. He carries his weapon of choice, a katana with a square tsuba, on his left hip, attached to his belt, but was also seen carrying it over his left shoulder with a long strap.

(Source: One Piece Wiki)