Who we can exclude from being the traitor:
Bakugo, Shoto, Ochako, Iida, Yaoyorozu, Kirishima.
Kaminari, Jirou, Ashido (fought the League and helped poison Giganto, and we saw Kaminari's thoughts, he loves Jirou + Ashido only remembered Giganto when she saw him in the forest).
Tokoyami, Mezo...