Natsume overhears whispers that a powerful youkai is reviving, later that evening he is trapped in a bottle as an offering.  Humans can’t see or hear him, so Nyanko-sensei decides to help the “best” way he can…
then, once his power returns, 
he must be fairly strong.
es un tipo que está recuperando sus poderes.
significa que, una vez recupere sus poderes, debe ser un youkai fuerte.