Responses
Depends on the extent of the contract. Worst case yes, best case no, but there'll be consequences. Caius is Kouichi's anchor in that world. If Caius dies, Kouichi can't stay, if Kouichi dies, as the summoner, Caius will lose half of his soul, etc you get the deal. The magic is Kouichi 'borrowing' Caius' power. That's what I understand.
I don't think my question is a spoiler but it maybe. WARNING!
If Cacius and Kouichi's soul are connected and Kouichi can now use magic. Doesn't that mean of one of the pair dies so will the other?