I'd like to propose Sayl the Faithless.
He's a Dolgan shaman (probably the highest ranking one?) that could have twigged to Mathilde's actions with the Kul attack and all the beef we bought from the Dolgan.
There's some interesting contrasts- he's called the Faithless for a reason, constantly betraying people for the upper hand and never devoting himself to one God, which contrasts with Mathilde's sincere devotion to Ranald and other causes. He lost his normal vision in a ritual, that means he now purely sees by his own version of Windsight (that's been mutated to give him a degree of precognition). He can cast from Azyr and Ulgu, so both Mathilde's Wind and one that she generally disdains. He has a living shadow called Nightmaw (formed from the bodies of three fellow apprentices in that ritual that mutated his sight).
Not sure about goal, though. Honestly, it could be Mathilde herself- Sayl collects the teeth of the many rival wizards and magical beasts he's killed, and Mathilde would make a worthy sacrifice for some bargain that he will inevitably renege on.
Part of me wants to suggest the Herrimaults from Bretonnia. I don't think they're ever explicitly stated to be Ranaldians, but they're professional outlaws that steal from nobles and protect peasants- yes, they're almost literally Robin Hood's Merry Men. Only problem is that she doesn't have much connection with Bretonnia.