S. is a homeworld gem
She was almost as big as Rose was and looked A LOT like her. I'm guessing she's a homeworld gem sent to infiltrate and spy on the gems and the best way to do that is to get close to one of them and since Garnet, the defacto leader is basically in a relationship with herself, the next best gem to get close to is Pearl and since Pearl was in love with Rose, posing as a human (who would suspect a human spy) that reminds Pearl of Rose would be the best way to do it.
Hell, S. may be a Rose Quartz herself.
In the zombie apocalypse, you don't have to be the fastest runner, you just can't be the slowest.