Castiel was good.
He loved his Father.
He loved his brothers and sisters.
He tried to be faithful.
He tried to be sinless.
He tried to be loyal.
Oh, how he tried.
Castiel was once an angel of the Lord. He was cast from heaven when it was discovered that he had become the ally of- and even befriended- a demon.
When his sins were first discovered, he was held down by some of his brothers and sisters, and they attacked his wings with blades, sawing off one completely and leaving the other one broken, mangled, and beyond repair. He was then exiled from heaven, left to wander the earth aimlessly.
Castiel is a quiet, sorrowful and bitter creature. He walks with his head lowered, eyes downcast, and a slight limp; the weight of his remaining wing presses down on his right shoulder, causing this limp, and serving as a constant reminder of his sin.