100% approve. Would strongly consider rolling something like a d4 to determine how the demon reacted (modified by species, because a palrethees isn’t likely to attack). So for a palrethees it’d be like:
1 - Furious, bitter anger. Every deal it makes for the next d12 weeks is entirely centered on ruining the fighter in the most humiliating and harmful ways possible. Leaving dead innocents (like children or loose acquaintances) in their rooms to frame them, assaulting loved ones repeatedly whenever they think they’re safe to cause them suffering, slandering them ahead of their travels, or even just trying to assassinate them.
2 - Cold respect and a reward for the fighter’s cleverness. Maybe money or even a magic item, that comes with huge strings. (It was stolen, its cursed, someone very dangerous wants it, etc)
3 - Indifference and pulsing its fear creating affect just to be spiteful and drive away the fighter
4 - Actual amusement and a decision to follow the fighter around. Their new “friend” would probably amuse themselves in the same way any demon would, so atrocities would follow the fighter indirectly, the demon would get to make deals with those the fighter wronged/conflicted with for their souls, and maybe the demon even “helps” the fighter on occasion. As seen through the guise of a demon’s idea of helpful of course. It might also just wander away after a week or so after it got bored.
Ah, the good ol’ “I’m not, but actually am, but not enough that I should get a raise, but I really would like one and less work hours, but I really need to stay longer because I’m so slow at everything I do and am terrible at focusing so I should really be working harder to give you your money’s worth, but you’re probably not paying me as much as you should be for that work in hindsight” theoretical with yourself and your imagined boss.