Fan Casting Diagnosed by the Audience in Sort by Tropes

This character's behaviour is bizarrely outside the norm — it's way beyond mere ordinary quirkiness. No reasons are given for the strange behaviour. No specific diagnosis is ever mentioned in the story. In fact, any resemblance to any real disorder is most likely unintentional; the character's symptoms are exactly the symptoms the writer wants them to have. However, perhaps the audience has an idea of what disorder they have, and try their hand at diagnosing them.