Life is not always easy when you fall in love with your tsundere childhood friend and want to marry her. Do your best, Juuzou !
This story happens after the events of th 34th chapter of Blue Exorcist. I guess, it has something to do with what happened with the Impure King, but I'm curious about Mamushi's reason to refuse the marriage.
Typo in first box on page 2. It should be - I don't know how it CAME to this. Typo in box in panel 4 on page 15. It should be - What we are doing now IS battle of obstinacy. Additionally, a battle is a one time event, but if they're doing it more often, then maybe use "war" instead of "battle". Also, I personally would replace "obstinacy" with "attrition".
There are other minor issues, but nothing that's really worth pointing out.