I'm re-reading and don't need to know who it is, just wondering if that plot thread is followed up on, as I've forgotten. edit:
it is.
Or is he referring to either Ren or a fighter he assumes will win a later match?
edit: By now, I've now re-read further, and I think it's clear who/what he meant. (I think) The answer is revealed well before
it wasn't one of the characters on my list, who had already advanced to the 2nd round.
He was referring to:
Saw Paing.
Katsumasa
had arranged for his fight with Karo Yoshinari to be fixed, by applying pressure to both Kengan members. Whether or not Saw Paing or Karo Yoshinari won, a representative for Katsumasa's faction was guaranteed to advance.