I haven't read anything of the original novel so this is just my assumption: the red-headed maid is the spy. She was sus already from the last episode since she appeared out of nowhere, this chapter confirms my suspicion.
Thank you for translating! Man, Nines is getting clingy. Also - isn't giving advice any more, so poor Yona has to carry the entire load for not having Melissa just run into things. Poor Best Girl.