Seishirou is “Sakurazukamori”, a group which consists of one person.
Sakurazukamori is a cold blooded assassin who uses Onmyoujutsu to kill.
He and Subaru are equal and opposite, the two
sides of the same coin: the Sumeragi clan supports Japan on the outside,
and Sakurazukamori supports Japan from behind. Seishirou has no emotions.
To him, humans are merely ‘things’ no different to any other objects. He
met Subaru when Subaru was a kid. He saw the kindness and innocence in
Subaru, and made a bet with him: next time they meet, Seishirou will live
with Subaru for one year and if Subaru can make Seishirou feel special
towards him, Subaru wins. It Subaru loses, Seishirou will kill him. It
was 10 years later that they met again, and Subaru has no memory of the
bet. Seishirou was really nice towards Subaru and tried very hard to have
some feelings towards him during that year. But Subaru’s kindness did not
change Seishirou, and Seishirou revealed his true nature, killing Subaru’s
sister in cold blood.