@Eclat123 Why not? The FL is a killing machine as well.
Like Paarthurnax said: "What is better? To be born good or to overcome your evil nature through great effort?"
Kasia and Ernon would understand each other, their problem and history, and could thus also support each other. If you take any other character, Kasia would actually be better off never talking about her past to them (even if she removed the other world aspect and altered the story so that it all seemingly happened in that new world). None of them could sympathise with her genuinely. However, Ernon basically went through the same: A shitty childhood, picked up by psychos, turned into a killing machine who just had to slay people when ordered to, no questions asked, all the while feeling like he (or she) had no other value as a human but that killing ability.
Besides, he could actually help Kasia to develop, as a mage. She already can use mana, so she fulfills the condition of having the gift for it.