Grandpa (real name unknown) is a Sim buried in the Goth Family graveyard in the Goth Home. He looks like Jeff Pleasant, with an almost bald head, and wears clothes similar to Mortimer Goth. It is unknown who he really is, or from what side of the family he was. His name suggests that he had children, possibly Uncle and/or Auntie.
It is possible that Grandpa is related to either Bella or Mortimer. Grandpa could be Milton Bachelor, though this is unlikely considering the difference in skin colors, facial features and clothes.