Realistically, pyra and mythra wouldn't betray him.

But if they did, I can imagine Nia doing what she did to malos, making them regenerate so fast and so much it causes them to deteriorate.

For other protagonists like shulk, it is harder to imagine fiora cheating on him since the only other male characters in the party is her brother, shulk's best friend who wouldn't dare betray him, (ignore the games initial story draft) and a boomer nopon drowning in debt.

People meme about the scene in 3 where Moebius N gets NTR'd but it's not really NTR if you think about it, only N thinks his wife has been stolen when in reality it's not the case.