So, two things I wanted to mention. First of all, didn't explain well in my post. When I was saying she'd find out his fate anyway, I was going under the assumption that she could see the details of his last few minutes with her magic or because he is her familiar. So his message was directed at her whenever she does look to him to figure out who killed her Familiar.

The second was basically a question of what we want to do next? A witch is above Mitsui's skill level. At least, what I'm assuming Karasuko would be at since she hired him. So it might be easier to send lower grade witches, creatures, or the Madness beasts at them in the next bit. Also thought of having random creatures/supernatural beings that they could run into. I've been considering having the two meet Kid's son in the next village they pass through. I think it might be too soon, though. I'm assuming Serephina will joined Kid's son if he's fighting the witches and searching for survivors. Mitsui's still only with her because he doesn't want her to be harmed. If he gets her to capable hands, he's still at a point where he would simply leave, satisfied that she can protect herself.