The Mystery Sim is a Sim only seen in memories of all Create a Sim and pre-created, single, young adult through elder Sims in The Sims 2. The purpose of Mystery Sim is to give young adults, adults, and elders love-related memories that are usually accomplished in the teen years, such as a first kiss, making it as a placeholder. Consequently, Sims 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.
- 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.