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In my opinion, Visa killed the witch (well, she doesn't look like a vampire), and Hosa is a joker with a madman's ND (neutral damage) x(.
If we believe Hosa and he's an analyst, then the Vistrian was a water elemental (well, I think so based on the art).
In general, I think we should look for the vampire now, not the mage (but that's just my personal opinion), because the vampire is much more dangerous right now...
P.S.: What a pity we executed Verovolka!