Joe is a Diancie. Joe is also not a dude. Joe got her name from declaring that she should be called "Jo-"(Japanese for "Queen" or "Lady".) While older than Movie!Diancie, she still possesses the same wanderlust and snuck out of the crystal kingdom to explore the world above, latching onto a random, directionless trainer so that she could stay in charge of where they travel while also avoiding trouble that would come from kidnappers due to being a caught Pokemon.

In theory, of course. In practice, people are still willing to try and steal her and causes her trainer a host of headaches. As you might guess, she has a haughty personality, but suffers from some poor judgment.