A happy ending for a ghost is to go to heaven or reincarnation of course :)
Well I do hope we get a little bit of fluff before that. I hope the line "I see you Aizawa-san" isn't said too close to the ending. That would be very very sad.
Ghost romances can have many different happy endings that aren't mostly bittersweet.
A. The ghost and human stay together until the human dies, then both move on
B. The ghost comes back to life and lives together with the human
C. The human dies and becomes a ghost and now it's a double ghost romance where they can be happy together
And there's probably some more.
