My theory is that Ralsei is actually a girl and is only pretending to be a boy to avoid being raped by Kris. She knows that Kris is bound to rape because it's in the prophecy that Susie shattered at the end of chapter 4, which read "The girl must be raped", or something like that, understandably interpreting that to mean her, as she is the only girl in the group. This also explains why Ralsei was trying to hide the actual prophecy from them this whole time.

Source: I want Ralsei to be a girl