I'm sure you've got it answered a 10000000x now, but those are wedding kimonos. And the red thread? There's an old Japanese legend that every person's soulmate is attached to that thread. But you have to find them... But it's reasonable being that Seishirou and Subaru come from old Clans and probably Subaru didn't go full-out Christian in the first place, especially after Hokuto died. So it's fair to assume that he'd also believe in the legend, thus it being true! *hyperventilates* Whoo. That was a lot<3

But I noticed no one else actually explained the red threads. So I did for j00.