Apparently, it's unreasonable not to trust a spirit / demon that nobody knows what it is, except that * it * knows how to kill people very well and looks like an undead.
Even without context, It would never come to my mind to blame anyone.
If Sera doesn't like Cole, because he's is we don't know what, it won't affect me in any way to be honest.
In my team, people are free to think whatever the hell they want about their mates. They are free to doubt, to ignore anyone, to be suspicious, to be friendly, to be in love and to come to me to discuss about their fears, their beliefs and their disagreements. As long as people are loyal to the group, I see no reasons to be upset.