Number Four
Simon is a Humano-Cylon agent who poses as a doctor caring for Kara "Starbuck" Thrace after her capture in "The Farm."
Simon tries to casually persuade her into bearing children but is unsuccessful. When his diplomatic efforts fail, he surgically explores her ovaries without Thrace's permission, either partially removing an ovary or its eggs. On orders from a copy of Number Six, Simon arranges a time to surgically remove Thrace's ovaries.
Before Simon could sedate her again and remove her ovaries, Thrace realizes he is a Cylon when in conversation he uses her call sign--she had never mentioned it in conversation. Thrace kills him and escapes from the Farm.
Another copy of Simon is seen just as Thrace exits the building, a more well-kempt copy in a dark business suit who greets Thrace just as she reaches the outside of the compound. Before he can speak further, members of the Resistance immediately kill him as they storm the Farm to liberate Thrace.
Simon was the first new Humano-Cylon model revealed since the Mini-Series, and brought the total number of known agents to 5. The later revelation of D'anna Biers' Cylon nature in the episode "Final Cut" brought the total to 6.
Note
Whether Simon has completely or partially removed one or both ovaries or just their eggs from Kara Thrace is unclear. The sentence that Kara overhears Simon saying to Number Six is, "Pending lab test results on the sample ovaries, complete removal will proceed tomorrow. If lab tests are positive, then subject will be moved to processing facility for final disposition." However, humans only have two ovaries, so it seems implausible that he would take a "sample" one, so perhaps the line truly meant "ova" (egg cells). It is clear that he didn't completely remove all of Starbuck's reproductive cells, as Starbuck escaped before "complete removal" took place.