DID is a horrible illness and they manage to capture various nuances whenever each came to the light, and seamlessly mixed it with the trauma he suffered with his abusers when he was most vulnerable as a kid.
When we go inside Arthur's personalities we can see how they were necessary to protect him.
That last personality might as well be the joker we all know from DC.
(This is what I perceived, I might be wrong and the characters I saw as additional personalities could be real people but if they are everyone acted strange around them and allowed them to do things inside a prison/assylum that do not seem ordinary)