Mystery Sim

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. (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♙

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.