Tsubasalover – really insightful observation on the source of Kyoko’s desire to please others!
One random theory that could explain Saena’s cold behavior is that Kyoko was the product of rape? Some women have a hard time loving the child that was fathered by someone who hurt them; the very image of that child is a trigger and would thus explain why Kyoko was abandoned. But I’m sure there is a reason, whatever that many be.
To be honest, I think it would be far more interesting to have Saena be an anomaly and not have a reason behind her behavior. Yes, it’s extremely cruel, and that's the point--she's a villain.