ChaoticKen said: They're known as the Ascended. Those who achieve enlightenment are capable of shedding their hats.
Suwako is an exclusive case, though. The hat is a parasitic creature that gained sentience when Suwako shed it.
Actually, it's a hat that's been a part of Suwako for so long that it gains sentience and it's picked up a bit of her power. Releasing the hat would only doom the humans.