Yomi is a Japanese term meaning "Knowing the mind of the opponent", and is essentially an intangible asset required in fighting games. It's the ability to know what your opponent is going to do, and act appropriately. Whether you achieve this by "conditioning" the opponent to act one way, and then acting in another way, or simply work your way into the head of your opponent, yomi is just that: the ability to know what your opponent is going to do.
Is blind, has three ears on each side, long dark hair, and a soothing deep voice rather similar to Sakio's from earlier episodes (english dub). Yomi wants to unify Demon World.