Kamui reminds Subaru of himself. This isn't surprising, why else would Subaru put up with him? Kamui also reminds Seishirou of Subaru, and this is only a little more surprising.
Subaru loves Seishirou, and vice versa. But they both want Kamui.
Subaru wants to protect Kamui's innocence, Seishirou wants to take it like he did Subaru's. They're not particularly good at sharing.
Kamui loves Subaru. Plain and simple, he loves him. That said, Kamui will never deny the rush that Seishirou gives him. Kamui might well love Seishirou, too.
Subaru and Seishirou aren't particularly good at sharing, but they try.
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