Maybe it’s a long way to go and I’m overthinking things...but I feel that this will have a bittersweet ending.
Like, she clearly likes him and probably he’ll come to realize the same.
But “facing reality” gets in the way (mostly, her having a much longer lifespan being a succubus than him being a normal human), and some things happen (his work gets approved and appraised, and a human girl start to notice him) so they’ll decide to not become a couple and just remain in each other’s heart as someone really important, with an increased distance over time.
EDIT : just to be clear, it’s NOT what I hope. Obviously I see them as the main -and only- couple till the end.
I just stated a “feel” that I’m having.