Sim was a mimetic simbiot of Commander Charles Tucker III, created by Doctor Phlox using a Lyssarian Desert Larvae. He was created after Tucker was injured and rendered comatose due to an encounter with a spatial anomaly in 2153. Sim's purpose was to provide Phlox with vital brain tissue, which was to be used to treat Tucker's condition. Sim resided aboard Enterprise NX-01, where he lived a lifetime in fifteen days.
Sim's development from a Lyssarian Desert Larvae began with cellular division in a growth tank inside Enterprise NX-01, following insertion of DNA from Tucker. Two days later, Sim had developed into a fetus, which was sensitive to light. He had been transferred to a birthing tank, which was usually covered by a sheet, and the lighting in Enterprise's sickbay was deliberately dimmed. Inside the tank, the fetal Sim was surrounded by a nutrient solution. Phlox expected Sim to be "born" the following day, and believed that once he aged to the approximate age of the real Charles Tucker, the clone could be a donor of the needed tissue and still live out the rest of his short lifespan.
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. To the surprise (and vague unease) of the crew, as he grew older, Sim 'inherited' all of Trip's memories from that age. Archer informed him of his true existence and purpose and Sim accepted it. As the days went by, Sim became part of the crew, putting Tucker's remembered engineering skills to good use. He even saved Enterprise NX-01 from a polaric field. Sim had strong romantic feelings for T'Pol, but she seemed somewhat discomforted by his advances. Meanwhile, Phlox discovered through further testing that he was incorrect; Sim would not survive the tissue transplant.
Wanting to live
Sim discovered that an experimental enzyme might extend his life beyond the fifteen days to the length of a normal human lifespan. Phlox told him that there was no proof that the enzyme would work and the time it would take to determine if it would work would age Sim beyond the point where he could donate the necessary tissues to Tucker, with the result that Tucker would die if Sim was saved or waited too long. Sim thought about stealing a shuttlepod to escape, and went as far as setting up the computer to aid in this escape, but he realized that if he did this, then Tucker would die and the Enterprise's mission to stop the Xindi might fail, causing the annihilation of mankind – not to mention that Sim would have to live the rest of his incredibly short life alone in space, "peeing in bottles" because the shuttlepod was not equipped with restroom facilities. With this in mind, he decided to submit to the operation. T'Pol kissed him goodbye and he reported to sickbay.
The ultimate sacrifice
Sim was painlessly killed by the operation, and was given a full-honors burial in space by the crew, including the resuscitated Tucker. Archer conducted a funeral service for Sim, noting that the entire crew was grateful for the contributions he made in his short lifetime. Sim was "buried" in a torpedo capsule with the serial number 2744586. (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, including those memories of Tucker's parents, he considered Phlox 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."
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")
In most scenes, Sim (as an adult) was portrayed by actor Connor Trinneer, since he usually played Charles Tucker. Child versions of Sim were played by Maximillian Kesmodel (apparent age 4), Adam Taylor Gordon (apparent age 8), and Shane Sweet (apparent age 17). An unknown performer appeared as Sim while he is depicted as a baby (both as a newborn and seemingly as an eight-month-old).
In the final draft script of "Similitude", Sim was commonly referred to (apart from in dialogue) as "Sim-Trip". In the same script, however, the character was twice referred to as Tucker. The first occasion was in the teaser, hiding Sim's true identity, and the second instance was apparently as a misprint that gave his name as "Trip" (rather than "Sim-Trip"), in reference to Sim giving Archer a smile in the launch bay prior to Sim's operation.
The "Similitude" final draft script also specified, "Sim-Trip has a light Southern accent." Concerning Sim when he is seemingly a seventeen-year-old, the script remarked, "He's clearly developed an odd self-awareness about his own, bizarre situation." To describe the adult, a note in the script commented, "From this point forward, Sim-Trip will be played by Connor Trinneer." None of the other actors who played Sim were referred to by name in the script.