Different anon, Dimi and Marianne is NOTP? I'm surprised I thought they're pretty popular! what turns you off on this pairing?
Also what got you to ship Hapi X Dimi , is it the shared past trauma with cornelia/cleobulus and knowing Anselma/Patricia ?
Answered the D/M stuff here since it was long!
With Dimitri and Hapi, it's more their personalities, but the shared trauma could definitely help bring them together.
Hapi is very relaxed and casual, and Dimitri often brings up that he prefers being treated that way and not like anything special. I feel like they'd be able to talk each other very easily with no worry about titles or any of that. Dimitri also does not care that Hapi is considered "dangerous" because of her curse and talks to her without worry. Their conversations are more two-sided as well, because Hapi is often so concerned about her curse in other supports that she worries about being near other people. Granted, I think Dimitri is also strong enough to deal with monsters if that ever did that happen (he thinks he is too lol, he's a cutie!), which I think would be/is reassuring for Hapi as well.
There's also the fact that Dimitri is very open to emotion, which Hapi doesn't express much of. I think it would be nice for her to have someone who wouldn't dissuade her from expressing her feelings.
I also think it's really nice that their past ties up into a nice bow. It's not just their past with his step mother, but their general backstories involving TWS as a whole. Dimitri has much deeper ties to TWS than most of the characters because of the whole Duscur story. Being able to defeat them, which in Hapi's ending with him she does, and he also has an active desire to help her get rid of her curse. I think it's nice that at least in one ending Dimitri gets some closure regarding Duscur, because normally in AM he doesn't really get full closure like he does in Hopes.
It's kind of a combination of like... their personalities matched with their ending and that at least what the story itself couldn't get to, at least one ending did fulfill. They seem pretty earnest around each other too, and Hapi doesn't really have that generally. I do think having someone around with similar knowledge to her is very helpful for her (and helpful for him too), because they have a very supportive friendship but I think they can also really help each other because of their respective histories. They can learn more and get more done with each other because they have that connection. Dimitri's lack of fear regarding her condition is also very important imo, because it's biggest insecurity she has and he's not bothered in the least.
Imo they just make a really good pair and I love the strong drive they both have against the same enemies and how that's part of what helped bring them together. In a way they fought for their ending together, and I feel like they could take care of each other emotionally very well. Other than Byleth, Hapi is also my other favorite canon potential ending for Dimitri.