The Promised Neverland is an anime and the girl with black hair is called 'mum' or 'mama' in this as she was one of the kids back there. Libby and Ava have numbers tattooed on there neck, Libby's is 5637 and Ava's is 5638. In this anime they live in a old farm house but they are used as meat to feed the demons there. Libby is 13 in this and Ava is 12 so your oc will have to be under the age of 15 because that's how the anime works so yeahhhh.