A Cloud
A Cloud is someone who shows up when you're happy and messes everything up. But later you realize they were the best thing that ever happened to you, even though you end up crying when they leave.
'They came in like a hurricane, changed everything, then left like a ghost.'
'I didn't think I needed them, but now I miss them like hell.'
'They made me laugh until I cried, and then they left me broken.'