He'd probably be pretty upset, since it would mean that Kamui didn't think Kotori and he weren't people he wanted to protect anymore. Fuuma would try to change his mind, probably. He'd want to try and get Kamui back to being the person he was before leaving Tokyo. So pretty similar to how Kamui is dealing with Fuuma being a Dragon of Earth.
What do you think it would have took for Kamui to become a Dragon of Earth?