They are the bodies of the characters; only their pixels and pink and corroded from being underground.
They can be brought back from the dead using a special spell:
Ima shinde kara jōshō. Hikari wa anata o uragitte imasu. Kurai wa, ima kite, kōkan-yō no dōmeikunidearu seikatsu no ato, kurayami wa anata o mamori, anata o ikusei shimasukara henkan shinakereba naranai.
Rise from the dead now; the light has betrayed you. The dark shall be your replacement ally, come now, return from the after life, the dark will protect you and nurture you.
Apparently they are beyond the stars's control, meaning they can do whatever they want to; they often search for bodies to move into once being resurrected. They can be brought back to their graves by throwing ink on them (it will corrode their pixels even further, even killing most)