Atmos is the commander of the 13th division of the Whitebeard Pirates. He is known as the Water Buffalo.
He is a large man dressed in armor. On each of his arms, he bears the tattoo silhouette of the Whitebeard Jolly Roger.
As the commander of the 13th division of Whitebeard crew, Atmos has command and responsibility over the pirates within the division. He is strong and is capable of wielding two swords.

(Source: One Piece Wikia)