Others: Kagami / GOLD____________
Kagami is quite possibly Seeu's best friend. Three hundred years ago, they fought side-by-side in the war; Kagami even died so that Seeu could live. Apparently, as we find out later, Seeu had been fated to die in that war but Kagami somehow took his place.
Kagami himself is somewhat odd; he sees the future that even the sages cannot see. As Kaguya's older brother, he has lived beyond the end of the world and knows what is to come. Therefore, he has been working to prevent the destruction of all the worlds. After the worlds are destroyed they begin again and repeat the cycle. Kagami is out to stop the cycle at some point and ensure the continued existence of the universe.
Seeu apparently shares Kagami's confidence; Seeu seems to know of his plan, for instance, and fiercely protects Kaguya. I would argue that Seeu feels that he owes it to Kagami to protect his younger sister.
However...Seeu is very obsessed with Kagami. I assume that he feels extremely guilty that Kagami died in his place. Seeu has created many dolls resembling Kagami; one of these is the current bearer of the organic weapon GOLD.
Things become even more complicated, however, because GOLD itself seems to retain some of Kagami's memories, if not his entire personality...