Who

If I recall correctly it was the current empress (the MC's stepmother).

But who is the man she fell in love with?

Oh, I think we still don't know. She (the ghost) doesn't remember who she was, so I think we will have to wait until Jaeshin finds out who the man was or until she remembers.

This is a fat guess but from the way she talks about the dude saying stuff like "his lordship" and "palace" I think it's ukes best friend who she loved

Dang that wouldve been just a great spice to the story!