I know Luigi is cowardly but in Luigi’s Mansion, he suffers from anxiety and has a phobia of ghosts as well(though he does control it at the same time). So please stop removing the category from him already. If you disagree then that’s just your own opinion.
It not just my own opinion. His phobias aren't enough to be categorised under Mental Ill. People sometimes do irrational things when they're scared and Luigi doesn't take that to the point where it's considered mentally unstable.