Sim was a mimetic simbiot of Commander Charles Tucker III, created by Doctor Phlox using a Lyssarrian desert larva. He was created after Tucker was injured after an encounter with a spatial anomaly in 2153 and rendered comatose. Sim's purpose was to provide Phlox with vital brain tissue, which was to be used to treat Tucker's condition. Sim lived a lifetime in fifteen days.
After his birth, Phlox became a surrogate father to him. He named him Sim and cared for him as Sim grew quickly to a boy, a teenager and an adult over just a few days. Sim retained all of Trip's memories. Archer informed him of his true purpose and Sim accepted it. As the days went by, Sim became part of the crew. He even saved Enterprise from a polaric field. Sim had strong romantic feelings for T'Pol. Phlox, who believed that Sim could be a donor of the needed tissue and still live out his short lifespan, found that he was incorrect and that Sim would not survive the transplant.
Sim discovered that an experimental enzyme might extend his life beyond the fifteen days. Phlox told him that there was no proof that the enzyme would work, that it would take too long to determine if it would work- thus aging Sim beyond the point where he could donate the necessary tissues to Tucker-, and that if it did work, then Tucker would die. Sim thought about stealing a shuttlepod to escape. He realized that if he did this, then Tucker would die and the Enterprise's mission to stop the Xindi would fail, causing the annihilation of mankind. He decided to submit to the operation. T'Pol kissed him goodbye and he reported to sickbay.
Sim was killed by the operation, and was given a burial in space by the crew, including the resuscitated Tucker. Archer conducted a funeral service for him, noting that the entire crew was grateful for the contributions he made in his short lifetime. (ENT: "Similitude")
Dr. Phlox was the man who created, and raised Sim during his first few days. Right before the transplant, Sim told Phlox that even with all of Trip's memories, he considered him to be "a damn good father." Phlox told him he had been "a damn good son."
Sim, like Trip, was attracted to T'Pol, but unlike Trip, didn't have the time to waste. He confessed his feelings to her, along with the ambiguity of their origins, to which she couldn't answer either way. Right before he went to sickbay for the final time, T'Pol said the crew, and she, would miss him, and then kissed him goodbye. Sim felt he "couldn't have asked for a better going away present."
Effects on others
Because Sim revealed his feelings for T'Pol to her, she knew, logically, that Tucker felt the same way. This led to a romantic liaison between T'Pol and Tucker months after Sim's passing. (ENT: "Harbinger")