This is actully a tough one. Being that theres a lot of evidence that Onedera could be the promise girl.

But there is one big thing that could say she isn't. Onedera states she was given the key from a boy. Raku has a flashback of the girl giving him the locket.

We can't totally cut her off since there is a lot more evidence of her being the promise girl. She could also be remembering incorrectly and she may have been the one that gave Raku the locket.

For now Onodera is still a possibility.