He'd probably be really f-ing confused at first, because what happened to being at CLAMP Campus. And then go about trying to find where Kamui and Kotori had gotten to, because that's what Fuuma does. He'd pester Kamui a lot to find out what happened, and if he couldn't get an answer out of him, he might try asking someone else. I think he's smart enough to put most of the pieces together. And once Fuuma found out, he would probably curl up in a ditch and die on the inside for a while.
How would Kamui handle Kotori coming back to Marina for good this time? AKA, how do you say "stay the hell away from your brother he killed you" in a good way? Or would he just outright tell her?