Alexios is an interesting case. The actor is Greek; he was born in Athens. But he moved to Canada as a three year old. I don't have nearly enough experience with Greek to tell a good accent from a bad one, but I can easily imagine how his accent could be different from someone who'd spent his whole life--or even childhood--in Greece.
Looks like the actress who played Kassandra, Melissanthi Mahut, went the other way-- born in Canada but raised in Greece. https://en.wikipedia.org/wiki/Melissanthi_Mahut Her dad is Québecois and her mother is Greek. I think I've heard her interviewed in English and her normal speech in English is much less accented than what she does with the role, but when she "accents it up" so to speak she's got the right base for it. Apparently she also voiced Athena in Immortals: Fenyx Rising.
In college (a million years ago during the first Reagan Administration) I hung out for a while with this particularly gorgeous young Greek woman (extremely Greek Orthodox, so there was never any question of "whoopee," alas) and I got to know a few other Greeks, so the accent became very familiar to me.
