In the past I always loved to portray a different "mental issue" with my characters, either that or to represent an idea.

I had played with Solipsism, Double personality, Psychosis, etc.

I've also been fond of making "troll" characters

Nowadays though I had calmed however I'm always trying something new or different than what you usually see in RPG.
Between the many I currently had this character whose memories had been completely removed (different to amnesia, as the memories aren't hidden, they simply doesn't exist) in a period that goes from childhood to late teenage. Thus this character's personality is a "plastic" one created from the mashup of her childlike personality with her teen one, but skipping a bit portion in the middle. Been funny to write as she can be quite irrational.