Short-lived clone grown from Trip Tucker's cells to save his life, who had to choose whether to sacrifice himself.
Sim was a mimetic symbiote grown by Phlox in Similitude to provide neural tissue for Trip Tucker, who had been left comatose by an engine accident. Living only about two weeks, Sim aged rapidly and developed Tucker's memories, personality, love of engineering and feelings for T'Pol.
When Sim learned that the transplant would kill him, he wanted to live out his natural life, and the crew struggled with the ethics of the procedure. Thinking of Tucker's sister, killed in the Xindi attack, and the stakes of the mission, Sim consented to the transplant, which saved Tucker but ended his own life.