Rebecca Goth is a non-playable Sim shipped with the game in Sugar Maple Coast, a neighborhood in The Sims 3 Pets (console). She lives with her cat, Buttons Goth. Her relationship, if any, to the Goth family is unknown.
Rebecca Goth was drawn to Sugar Maple Coast due to the stories of her family there. This suggests that she could be a distant relative of the late Timaeus Goth. She can be made playable by asking her to move in or marrying her, though she is Unflirty so marrying her is difficult.
According to her biography she has a great-grandfather, who could be Mortimer Goth; this would make sense because Alexander Goth is married in Sims 3 Nintendo. Timaeus Goth, a dead sim buried in the Goth Family Cemetery, could be Alexander's child, making him Rebecca's father. Angela Gilman is another dead Sim said to have been married to a Goth which could have been Timaeus, which would make Angela Rebecca's mother. However, her great-grandfather could be Timaeus rather than Mortimer, depending on when he died or where The Sims 3 Pets console was set in time. (In turn potentially making Angela Gilman her great-grandmother rather than mother.)