A mimetic symbiote of Commander Tucker, Sim was created by Doctor Phlox from a Lyssarrian desevae. Tucker had been badly injured, and Phlox hoped to use neural tissue from the short-lived clone to repair his injuries. Unfortunately the amount of tissue required was far too great for Sim to be able to survive the procedure. Captain Archer, in need of his Chief Engineer to complete the Xindi mission, authorised the harvest anyway and Sim was indeed sacrificed. He was given a burial in space with full honours. [1] 