The characters are considerably different from one another because of their ages, and it's what drives the story forward. Female MC is very "babyish" which isn't surprising due to the time period - but made for a slightly uncomfy read (she talks in a baby voice in one of the panels) and she is constantly jealous, pouting, or whining about something.
Male MC makes a promise to her father not to make a move until she turns 20 but does not communicate this. This leads to frustration - the female lead is always wondering and stressing over the fact he won't reciprocate or initiate stuff. Though judging by their interactions in just a few days of marriage, I doubt he would stick to that promise. I also doubt she would be fine waiting 4 years.
All that said it wasn't a bad story. I think the length was fine for what it was and I liked the mystery surrounding the male MC. The art was good (same art as Horimiya). There isn't much that happens - just random things thrown in to create minimal conflict and rile up female lead. I think the story would've been so much better if she was older and they could've really ran with a similar premise. The characters have good chemistry and the reasoning behind the male lead wearing a mask all the time, youkai etc. I can't really complain though because the age gap is the driving point of focus in this story.
I give it a mixed review because I know people will enjoy this short story, and others may be completely turned off by the premise.
