Mystery Sim

The Mystery Sim is a Sim only seen in memories of all Create a Sim and pre-created, single, adult and Elder Sims in The Sims 2. The purpose of Mystery Sim is to give adults (along with young adults) and elders love-related memories that are usually accomplished in the teen years, such as a first kiss, making it as a placeholder. (A Sim cannot use any Kiss interactions other than First Kiss unless he or she has a First Kiss memory.) If one creates a family in CAS but is missing a parent, the Mystery Sim takes on the parent's role in the memories. Oddly, Mystery Sim also appears in the memories of some Maxis-made Sims, such as Ajay Loner and Don Lothario. The mystery Sim appears as having a very dark skin color and due to the mystery sim's saturation being dark, it is hard to notice the eye color and the lipstick color.

In-game it appears that they are wearing either a pink or checkered shirt.

Trivia

 * The face of the Mystery Sim is intentionally androgynous, allowing it to be used as either a male or female Sim in a memory, depending on the case, as it uses a hairstyle very similar to one of the default male hairstyles, a female shirt, and its face is a mixture of a female and a male face.

A Shocking Discovery
If you look at Mystery Sim in just the right light, you can see that he or she actually has a proper sim's face! After photo enhancement, this is what I found: