I don't have actors for everyone, but if somebody suddenly bought the movie rights to Errant and asked for my suggestions...
Millie Bobby Brown as Rekki Lune - The only realistic choice on this list, honestly because of the time movies or series take to make, she'd be about the right age to play her. British actress with an unconventionally pretty face and distinctive straight browline who's used to action scenes and has a lot of charm and attitude.
Dev Patel as Subo Talwar - I said to my punjabi partner "who's the south asian equivalent of Chris Pratt?" and they said "Oh for Subo? Definitely Dev Patel, that guy from Slumdog Millionaire." Yeah... he's perfect.
Robert Pattinson as Urien Ellis - Not only is he incredibly gorgeous, but he's also good at playing intensely unpleasant assholes and sociopaths. Absolutely perfect for Urien.