My favourite Sim has to be Peach Blossom.
I created her very early on, on release day. Ever since I have found ways to have her in many of my saves one way or another. I don't know why but I find her so sweet, and she always turns out to be so much fun to play.
Her original story was that she grew up in Hidden Springs under her very strict parents. But she was creative and quirky and never fitted in their shallow and lavish lifestyle. They had high expectations for her and she just wanted to find her place in the world on her own. So Peach moved to Oasis Springs and cut off all communication with her past (at least for a time).
Here is Peach Blossom base game style.
https://i.imgur.com/3vdJBwo.jpg
https://i.imgur.com/aE2QNyh.jpg
And here she is in my current rotational safe.
https://i.imgur.com/yjcepI4.jpg?1