Sierra Zhang is one of the pre-made townies in Sim State University from The Sims 2: University. She can be made playable by befriending and asking her to move in, or by marrying her. Like most pre-made townies from University, Sierra's skills are usually randomized at the start of the game.
Even though Sierra has black hair, her natural hair color is actually red. She has the same facial structure as Trista Shaw from Strangetown. Sierra shares her name with a burglar from Veronaville. Her name is randomized when Sim State University is generated. The name "Sierra Zhang" is only referenced in the game files.
Strangely when editing in SimPE, Jared Starchild recognizes Sierra as his girlfriend, but the relation isn't mutual, as Sierra doesn't even know who Jared is. The reason for this unrequited relation is unexplained, but there are a few other Sims from University that have these sorts of quirks.