Shark was the major assassin and war-leader of the Bloc Rebmaquois, a powerful Rebman warrior and shapeshifter who wielded an Outsider blade like a hole into the night sky--the Sword of the Void. His preferred form for combat was a kind of human/shark hybrid.
He was slain by Lucien during an underwater battle, but passed his soul into the Sword of the Void and sent it out into Shadow to find Dolphin at her time of greatest need. Thus, he appeared at the battle where Dolphin was captured by Motoko, Jacqueline and others.
Thoric stabbed the Sword of the Void into the Logrus-charged Dolphin at the climax of the fight; the reaction between the energies of the Outsider blade and the Logrus cast Shark's soul out of the Sword and presumably destroyed him forever.