From what I remember, because it's been a hot moment since I've watched one piece- Robin can't turn into a demon, but she can make a demon clone, so I guess they're just playing around with canon here.
From the plentiful jargon I keep throwing around I'd never imagined "handler" to be the one that gets people.
A tsundere handler is somebody who handles tsunderes, and in Columbina's case, very proficiently.
^That's what I was thinking you meant. I said partner but maybe trainer would have been better, I was thinking partner as in someone in a relationship with a tsundere that has to handle, manage, and deal with their partner's tsundere-ness lol