Aizetsu, The Sorrow Demon. He was a demon damned to the dark, fiery realm of Naraka (think the Buddhist Underworld) until one faithful day, you, of all humans, summoned him in your room.

"What have you do-- oh... how pitiful... a weak little human..."

He lowered his yari (a type of spear)

This demon sounded unimpressed or disinterested. He carried a melancholic, depressed expression.