Board Thread:News and Announcements/@comment-25285156-20150130030202/@comment-25094659-20150302124112

KidGleeful wrote:

All we know is that Soos and his grandmother share the same last name. His grandmother most likely got her last name from his grandfather.We also don't know for sure if Soos' grandmother is from his mother's side. She could be his father's mother, therefore passing her surname onto him. Okay, but where do we hear that they share the same last name? I don't remember when that was, and it seems kind of important to this conversation. :P

The way Soos's grandmother talked about his father made me think that she was Soos's maternal grandparent, although it's possible that I'm wrong, or that Soos's parents never "married" and Soos stayed with his mom, adopting her last name. Although I wouldn't bet on it too much in a kids show, even with the way Gravity Falls is becoming les and less one of those :P