Sewer King is a fictional character in the Hey Arnold! TV series.
Little is known about the Sewer King's origins. He mentions his name was Stan and it is implied by his hard hat and knowledge of the sewer systems operation that he was once a sewer worker. For unkown reasons he has chosen to live in the sewers where he declared himself the king. In the episode "The Sewer King", Arnold dropped his Grandpa's watch down an open manhole and he and Gerald go down into the sewer to get it back. Sewer King takes it and calls his "royal icon" and refuses to return it. Arnold plays him in a series of chess matches, all of which Sewer King loses but continues to ask for a rematch. Arnold just takes the watch and he and Gerald escape the sewers before the Sewer King can stop them. Grandpa ended up dropping the watch down the sink drainpipe, revealing he has numerous spares to replace it. The Sewer King manages to get the watch. In "Return of the Sewer King", Arnold and Gerald suspect Sewer King was turning off the cities water every night between midnight and one o' clock in the morning during a heatwave. Sure enough this was the case, as he was using the water to create a pool for his pet alligators. He set his largest alligator on Arnold and Gerald, who both recognize it as Harold's lost pet baby alligator and manage to get him to calm down. They convince Sewer King that the alligators would be better off in a zoo then with him.
The Sewer King seems to display traits of a narcissistic personality disorder, considering his disregard for the safety of other human beings or their property, considering he took Grandpa's watch, sicked one of his alligators on Arnold and Gerald, and would turn off the whole cities water to make a pool for his pet alligators. He also shows signs of delusion considering himself a "king" and Grandpa's watch a "royal icon", all of which show his tendency towards narcissistic personality disorder.