Mafia Sans wears an all black suit along with black shoes, black gloves, a red tie, and a black fedora. Most of the time he is depicted smoking a cigar.
Mafiatale Sans is known to be highly dangerous when fighting; however, he is usually lazy and carefree, despite his demeanor. He is a part of the "Skelefamily", a Mafia family. Frisk became a member of this family as well, before becoming Sans's, Papyrus's, and Gaster's "Boss".
Originally, before Frisk came along, Gaster was the boss of the family. He found it to be tiring after a while, however; he offered Sans the position, but Sans declined. By default, Papyrus became the boss. When Frisk was put under the family's protection, Papyrus was overjoyed enough to give them this role.
Mafia! Sans's weapon is usually a tommy gun or a pistol. Papyrus uses a sniper rifle (he's a sniper, he's going to use one, I'm pretty sure), and it is unknown whether Gaster uses a weapon, though it is speculated he does when fighting.