@Kayriel
She causes a lot of issues.
1, Its another traveling companion if she goes with him, but it is unlikely she can stay with him for long since she is a proper noble. Even if she wanted to, she has a role and an obligation to serve her citizens as well.
2, She will become a tie to the baron's territory. Meaning everyone will take it as a bias towards them. Not a great thing to have for a traveling merchant and his enemies, which are demons.
3, She is a lot of work. While she is not bad at fighting, and she has her artifact, she still would need further training and practice. Plus food for her as well as other needed things would add to his costs, not that he does not have the money, but appearing to make enough to cover those added costs would be much more difficult. If people figured out that his expenses outweigh his gains (like they should since he really does not sell anything since he mainly just uses his merchant title as an excuse to travel), more people would try and rob him, even more so since she would be an established noble. And who ever heard of a poor noble?
4, She may like him, but he does not necessarily like her in the same way.
5, If she becomes his lover, how would others see her? She clearly was not his first traveling companion, and he already has a few "of age" women with him. She would not become his "head/first/main wife" which could be taken as an insult to her, since she is actually the only recognized royalty traveling with them.
6, They met like what last month or so? Kind of fast paced for a life committing marriage. Remember he is originally from our world, which marring someone you just met is usually consider a bad thing, or at least is not seen in a positive light. So he may be more hesitant from that bias.
7, He still wants to get out of the game and go home if he can. It may not be his #1 priority at the moment, but it still is there in his future plan.