It's extremely funny that the entire foundation for Rean's arc in CS3/4 happens entirely off screen until they made a shitty anime years after the series had finished, long after everyone stopped giving a fuck. What were they thinking?
I assumed it was the usual bullshit side content released as novel/audio book for Japs only. Not that it was very important, being generic morally grey conflict that Rean somehow managed to pass literally without killing anyone.