Two people bonding over their lack of desire to be in the palace made for a fast friendship and that was the case for Cian and Quinn.
Even though Cian was being used as a convenient tool for the King and did so without complaint if it benefitted what was left of his people, he still didn't want to be there. He would be respectful but couldn't hide his emotions.
Many Eirenguardians would call him a traitor for even stepping foot in Castille without ill intentions. Yet he wanted peace just as much as Castille did. While Eirenguard was tired and willing to lose more to prove a point. He didn't want the same. He believed his people could be saved.