Yo it's obvious the author's trying to make us think the other traitor is Shibe. I'd shit bricks if it's actually the person Yuiichi trust the most right now, Mikasa Tenji. The admin also hinted that the second "traitor" is not the usual betrayal. Perhaps the second traitor's past or action unknowingly pulled the group into tomodachi game. Yuuichi might be the one, because they bring up Shiba Taizen and Katagiri Yuka so much.