Previous posts in the Harry Potter DSM: Cluster A Personality Disorders, Cluster B Personality Disorders
Avoidant PD
Things really break down here – JK didn’t write Avoidant. (Um, I don’t know what else to say, guys. Here: have a video of a pigeon chasing its own tail.)
Avoidant Criteria: Social Anxiety Disorder on steroids.
Dependent PD
Dependent Criteria: Oh, Merope. As soon as her family was imprisoned, she love-potioned a man into caring for her (“Urgently seeks another relationship.. when a close relationship ends”). And when he left her, she let herself die rather than live on her own (“Is unrealistically preoccupied with fears of being left to take care of herself”).
On the other hand, her situation seems like it would call for a rule-out of just about every mood and anxiety disorder there is. And I’d love to know how she would’ve functioned if she’d had a single caring source of support in her life – but she was never in a situation where she was encouraged to be independent, so it’s hard to claim that these characteristics were fixed.
Obsessive-Compulsive PD
Harry Potter: “Percy’s enjoying work, then?”
Ron Weasley: “Enjoying it? I don’t reckon he’d come home if Dad didn’t make him.”
OCPD Criteria: There’s a big difference between OCD and OCPD. You won’t find Percy obsessively washing his hands, or double-checking the stove 500 times. Instead, he works excessively and is overly dedicated to rules and regulations. Does he get so obsessed with doing it right that “the major point of the activity is lost”? In my highly unscientific opinion: I’ll bet you anything he does.
Also, in a fight between “family” and “work”, it was pretty obvious that work won, since he abandoned his family for it. In fact, he stuck with the Ministry until Voldemort was making a final stand at Hogwarts. So basically, this conflict only began to seem like a bad plan when it became less “family vs. work” and more “family vs. THE DESTRUCTION OF THE ENTIRE WORLD OH GOD”.
And that, my friends, is what they call “rigidity and stubbornness”.
(A small note: Every time I hear anyone pompously talking about their important research in the beta subunit of an arcane protein, I think of Percy and his pride at being put in charge of a Very Important Report on Standardizing Cauldron Thickness, as well as his total outrage that nobody seemed to take it seriously.)
Action Potential 
Only tangentially related, but are you excited about Pottermore? Or are you too cool for that?
PS I am not.
Dude, with the addition of all her encyclopedic notes about the HP world, there is no way this won’t be awesome. I’m so there!
I mean, were it not for his supportive friends, Ron is on his way to Avoidant:
A pervasive pattern of social inhibition, feelings of inadequacy, and hypersensitivity to negative evaluation, beginning by early adulthood and present in a variety of contexts, as indicated by four (or more) of the following:
(1) avoids occupational activities that involve significant interpersonal contact, because of fears of criticism, disapproval, or rejection — Do you remember the whole quidditch ordeal???
(2) is unwilling to get involved with people unless certain of being liked — Um, how long did it take for him to make a move on Hermione?? Oh wait, he didn’t — SHE did!
(3) shows restraint within intimate relationships because of the fear of being shamed or ridiculed — see 2
(4) is preoccupied with being criticized or rejected in social situations – Insecurity of this character runs deep.
(5) is inhibited in new interpersonal situations because of feelings of inadequacy – What was the deal with that date to the ball anyway? Just ignoring her the whole time?
(6) views self as socially inept, personally unappealing, or inferior to others – Remember what he saw in the mirror? Or how about what the horcrux said to him?
(7) is unusually reluctant to take personal risks or to engage in any new activities because they may prove embarrassing
— well this one he was sort of forced to overcome…
Yes!