Reiryoku is a power aligned with the spiritual sensitivity and willpower of the user. It is used by Shinigami and other spiritual beings to provide power for their various abilities. You start out about as strong as a Shinigami in training or newly-formed Hollow.
With Reiryoku, you're capable of seeing ghosts and spiritual beings, affecting spiritual beings and sensing the spiritual power of others.
With the right training (and one of the races picked), you could stand in the air while keeping still, running at the level of supersonic speed or even light speed, living for thousand of years, manipulate Reiryoku to heal, defend and for powerful attacks of sheer spiritual energy.
You're capable of emitting Reiryoku as "Reiatsu" a form of intimidation capable of even making people much weaker than you die by your spiritual pressure.
(While capable of many things on it's own, this power works best with one of the races below picked.)