I am not that far into it and I already have a theory.

So I episode jumped just for one of my short writes about Yuto's death. (sorry) Yuto and Yugo called each other spawn of fusion. Both look like Yuya plus number four. I only got a glimpse of him and I really have no idea who exactly he is. Especially the type of summoning he does. So there is a Yu in each dimension that look like the other and does that type of summoning. Before I get to my point.

Thanks to AuthorUprising36 I know Yuto's soul rests in Yuya and for the photo. Now, going back to DM manga when we first meet Shadi he say not only has he never seen a person with two mind rooms. Later it is stated in some way that it should not be possible for a person to have two mind rooms. The Millennium Puzzle is what held Atem during their adventures.

So basically is the Yu's the spawn of fusion? And do these four hold fragmented souls? Is that why Yuto is in Yuya? Is Yuzu key to awakening the spawn of fusion? What about the other hers? Are they too a key?