As it turns out, she was right. Seishirou turned out to be a tightly self-controlled sociopath, which made him a perfect assassin for her (which she calls “sword” to differentiate from Notte’s “knife”).
This combination of responsibility and cold-hearted killer made him – so Ravena thought – the perfect person to keep track of Ravena’s pet project, the seer Jonathan. As it turned out, that combination also doomed him.
Seishirou made the decision to act on Jonathan’s behalf instead of Ravena’s own. Even he doesn’t know why.