He is a small light purple slug with a light green underside. He has yellow and black eyes as well.
At the end of the episode, the elder starfish finds a coffee stain revealing Gary to be Sluggo's son, and the true and rightful king of Bikini Bottom.
He is never mentioned or appeared in an episode, although he is seen during the episode on the poster. He and Miss Wilson might have died since SpongeBob owns Gary, but it is unknown why if he did die because Gary seems young.
Given that the right to become King was passed to Gary, rather than Patrick, this implies that Sluggo is the older brother of Herb.
His surname is possibly Star, as that is his brother's surname, but it is equally likely that he changed his name to Gary Wilson, Sr., since his son's name is Gary Wilson, Jr.. It is unknown why if he did change his name.
It is possible that Sluggo was adopted, given that he is a mollusk, yet both of his parents are starfish.