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.) In the case of married Sims created in CAS, Mystery Sim is replaced with the Sim's spouse. Appropriately, the Mystery Sim has no face, and is just a silhouette. Oddly, Mystery Sim also appears in the memories of some Maxis-made Sims, such as PT9 and Don Lothario.

In-game you can slightly see he/she wears a checkered shirt.

Trivia
The face chosen and given to Mystery Sim looks more androgynous, due to the fact that the identity of this Sim remains unknown, and due to the fact the Sim can be either male or female, depending on the cases.