Board Thread:General Discussion/@comment-31344849-20170219160624/@comment-34645353-20180305225958

I think Goner Kid might be his sister, but I thought of another theory (which I found out is actually a theory some people believe).

What if Goner Kid is from an alternate timeline where Undyne didn't save Monster Kid, and they fell?

Instead of dying, however, there SOUL lingered and they became Goner Kid. So there is an alternate timeline now where Monster Kid doesn't exist anywhere. If you have the right fun value then your timeline is the one where Goner Kid is seen.