The age difference isn't that big of one. Since both of them aren't even legal age, it's kind of more of a grey area at the moment mostly depending on the statutory rape laws and how they were designed to handle this sort of thing, and becomes sort of questionable once she's definitely legal and he's not. However, it's also an arranged marriage fiance type deal at most anyway so that's actually pretty irrelevant. It's not like they're meeting up to start actively hooking up or anything. They're literally meeting just to... well, meet each other. For future reference rather than anything more immediate.
If they're following Japanese law they'd have to wait until he's at least 18 to get married. Do they even have to is kind of a valid question though, the laws apply to humans and human they ain't so they could maybe monster-marry whenever they want and get it on paper with the human side of things when they can. Not to put too fine a point on that, but they're not human so our whole deal is kind of irrelevant unless they want it to be I suppose.
TL;DR They're not only fictional characters in a story that is super unlikely to depict much if any underage sex in any form, they're not even actually fictional humans.