I've a penchant for criminals with inflated egos and plotting obsessions, and their descents into madness.
One of my first impactful characters, albeit short lived, was a businessman tiefling of Beshaba by the name of Kaen Piccoli. He was smarmy and suave, with white hair and deer antlers, until he got angry and his glowing eyes filled the entire room with an unholy glow. With the severing of his hand by his brother Able, He went steadily insane because his visage of "Beshaba's Chosen Son" was sullied by a missing hand, and went on a killing spree of friends and allies with his protege Caleb aka Flea and a friendly Maskarran.
Galmor of the Wyvern, My Werewolf, was my flawed take on Evil Superman. Using the Kill Bill 2 Quote "Batman puts on a costume, Superman is always superman, he puts on a clark kent costume to blend in, acting weak..." etc, Galmor was a natural born lycan who spent a good deal of time doing the sociology stuff of humans, in hopes of finding a suitable, burly, strong in the sense of a malarite city for him to reap infected children from. Lower was his choice, and he joined the Nancies. That being said, A fellow by the name of Horadin drove him mad. A human cleric who outed him, and was slain before he had a chance to kill him after an awesome IC letter forum correspondence, his personality was split between the Natural Lycan "The Fury" and "The Wisdom" that he had inately as an intellegent creature.
Every single one of my characters has a trigger, something that if it happens, will drive them slowly bat shit crazy, and eventually turn them into a rambling maniac who licks the blood from his own stump of a hand while giggling (<_< Kaen did it. It was one of my favorite RP'd moments on EFU, belated <3 to those involved). And I think that is what always made it interesting.
Be you an LG Helmite paladin who is highly militaristic, Or a simple goblin who is always finding his surroundings odd with his strange philosophy, Mental Stability in a setting like EFU is something I always loved.
Its dark. Its dirty. Its gritty. Its in the middle of the earth. 60% of people are escaped from slavery from the worst hells the human mind could imagine. It makes sense it would take a toll on one's sanity over time. Selunites don't have all the fun, and making a character slowly go crazy adds a certain something to a character I've seen over time, that really makes them feel more alive.
Not all of my characters have gone crazy, but those who did really stand out.