If you'd rather just know and be done with it, the real traitor is Galinn. Soma will react by executing him and Birna will join you and the Raven Clan.

Which Soma Advisor is the traitor?

Lastly, you will have to return to Soma. You can present to her all the evidence you found and information about her advisors, and make a decision. The traitor is Galinn. The game may confuse you with the fact that Lin had yellow paint, but it was stolen and used to help enemies infiltrate the settlement.

Which of Somas advisors betrayed her?

Galinn is the traitor. Select him and Soma will call a meeting of her trusted advisors, killing Galinn despite his protests – he lies through his teeth even in death.