Scratch wasn't talking about the rat when she said we still have HER. Tardust says "No" when Scratch says her. This points to the rat actually being a male character. Tardust clearly meant the rat but from listening to it it seems clear that Scratch thought he meant Lizzy, as in "what do we do with Lizzy". Also they say they haven't heard from the rat for a while and while that does point to it being someone who left the tower in that time, the fact that it is heavily implied that the Rat is not female leaves me to point the finger at either Angel, Kalani or Datu.

Datu was clearly disillusioned with the tower during the end moments of the war and had a death wish, and was in his room the whole time. It wouldn't surprise me to find out that Datu has a second radio or something like that in his room and maybe he had heard the mallers talking and reached out. In the mental state that he was in it would have been easy to get information from him.

Angel clearly is either hiding something about his past or at least not offering up important information. Scratch seems to recognise him and he knows how to hotwire cars. While knowing how to hotwire a car or knowing a completely fucking crazy bitch like Scratch does not neccessarily make you a untrustworthy individual or likely to betray people, it does point to a connection there. If you also remember it has been made clear at points in the series up to that point that Michael and Angel didn't respect his leadership abilities (although a mutual respect had started to develop at this point) and you will also find that one of Lizzy, Pegs or Rilly (my mind is blanking on it right now) had asked him where Michael was to find out if it was ok to do something and from memory Angel was annoyed at that. A combination of a lack of respect for him as a leader and this connection with Scratch and implied past point to him as a strong subject.

Which brings me to Kalani. Basically, when Kalani first entered the tower he seemed very hesitant to trust Michael and co and I think that if he had access to the radio he would very likely want to reach out to other survivors to see if anyone was out there he could go join instead and then all the mallers would have to do is convince him that they would have better leadership and that would be all she wrote. Kalani proved in the arena that the only thing he cares about is his survival, not that I can blame him, while also proving that he was very quick thinking on his feet and he may have decided in a brief moment that the mallers leadership would give him a better chance of survival. I also, if my memory of the scene is correct, don't remember hearing him during the party scene which would have been the perfect time for him to be on the radio.