Well, first off, you’re not the fictive in that case - your headmate is. You’re just hosting him. In the old terminology you’d be considered a soulbonder, but since the term soulbond has been abandoned in favor of fictive I’m not sure what the host is called in the new parlance.

As for whether things your headmate did in their life/dimension will have an influence on you, I’d say they probably will on some level. Not necessarily as simple as you being more likely to do things they previously have done, but imagine if they were simply your long-term roommate - having a roommate who you knew killed someone would probably have an effect on you. And headmates add a whole new layer of intimacy, and complete lack of privacy, to the equation.

As for the deal with the devil thing? I would be doubtful of that influencing you in the here-and-now unless you’re also hosting the devil from their dimension. And even then, it’s not like they would have magical devil-powers to enforce it here, they’d just be another fictive in your head. ~Wrath~

Edited to make rebloggable.