Seeing as Ralsei knows about (You), keeps secretly talking to Kris about secret shit over and over again, and knows the Knight isn't doing this just to destroy the world, it's not too big of leap of logic to assume he knows that Kris and the Knight are working together
If anything, it would be weird for him not to know. Why wouldn't such a major plot revelation be covered in the prophecy? If such a thing was unforseen it would actually make what Kris is doing a break away from fate, rather than continuing down the fated path like we assumed

>>712877673
If the Knight wanted people dead, they would have just killed Susie and Ralsei right there and made off with both Toriel and Undyne. They had perfect opportunity to. They could have even just injured Susie. They didn't.
It's been made very clear that that's not what the Knight is out to do. Things would be much simpler if they were going after their goals by any means necessary. There'd be bodies already.

>>712598058
What did you want him to say?
>Hello my name is Ralsei you are prophesized to stop the end of the world but in order to do that you must form a lesbian romance with that annoying girl in your class
>Oh also that guy standing right next to you is also a prophesized hero but he's also working with the mysterious masked figure who's working to destroy the world but may not actually be trying to destroy the world and he's also working for that girls mom and he's also possessed by a Demonic spirit who controls his voice and actions but sometimes he resists it and his own wishes and actions take precedent but sometimes not
>Anyways don't try to stop anything Kris is doing because if you do it could mess up the prophecy and also Kris isn't actually a bad guy and probably isn't doing this for bad reasons you just have to trust me and him and trust that everything will work out in the end
>Oh and there's a man who exists in the seams if reality and sometimes gives Kris an egg
Susie isn't ready for the full truth

The knight is the good guy