Look, I love a good harem ending as much as anyone but you absolutely do not owe them keys or "keeping things fair" if you've already made your choice.
Villain kills the fox: MC does nothing
Villain eyes Nina, suggests selling her: MC does nothing
Villain keeps trying to appraise MC: MC does nothing
Villain demands MC buy him lunch: MC does
Villain attacks MC: MC fakes a teleportation and hides
Villain attacks the innkeeper's daughter: MC...
Ah, what a wonderful ending, when two people who are extremely mentally ill and liable to forget most of their life or go completely catatonic at any moment decide to be responsible for each other's safety for the rest of their (almost certainly short) lives
Merchant collapses to his knees in tears, clutching the bottle of wyvern's bone marrow fluid to his chest while blubbering his thanks to the MC
MC: I am probably not good at understanding other people's subtle emotions
Wait… your grandmother (presumably an elf) used to travel with the hero "summoned from another world" (presumably human) and you… are a half-elf.
Feel like there's a tiny piece of the story that you aren't quite saying here.