Okay, thought about that idea. Since Tobirama had non-"main branch" Senju leave the clan and form their own families, and even Nawaki and Tsunade gave up the "Senju" name, then it'd make sense that this SWK would be their cousin, at least? So you could say that he's a grandchild of, say, Hashirama, and then also make him Madara's grandchild too, but unacknowledged?
