i'll take my shot and say that Kendra might be one to get uncorrupted. i always found that the interaction between Kendra and Maryen wasn't simply a "good and evil" interaction, but more like an actual argument on why they should or shouldn't kill people, while Maryen's point is a valid "because it's wrong" based on morality, Kendra's point is "because they're assholes" that is probably based on personal experiences (not the best, but still better than the "because i can" that we got from a Homaspernia dragoness during Elliette route), and now the mc (us) ended up in the middle and he must take a stance.
Route 1: we side with Kendra since he saw how awful people can be and he ends up being as horny as Kendra and that's the end of the story.
Route 2: we take Maryen's side and show her that not everyone are selfish people that only care about themselves, convince her to change and to start helping others (how to show her that not everybody are evil in a place like Homaspernia? idk, maybe wolf-kun will come in clutch somehow)
I highly doubt that Kendra would change to better and I cannot fully agree that she kills people simply due to her biased opinion: "because they're assholes". No, she clearly can define kindness from evil, that's why MC not only survived but became more than just her slave at the end. Kendra does relish her power, which is way above any human-sized person, but still has her boundares: she doesn't simply turn into a blood bath every place that she visits, and can actually show some appreciation to the "weaklings". After all, she was raised in Keisiema. (Yes, yes, she's destroying neutral towns for fun, blah blah. Everybody knows that they are filled with outlaws, adding Kendra's general contempt to normal-sized people to that and it becomes justified enough for her to do what she does)
Kendra is a demoness, she chosen this way of life long time ago, she does not seek redemption, so there is nothing to "uncorrupt", but her sister still mean something to her. She was originally planning to take Maryen and their dad with her after a murder. She even confess to that when MC confronts her, asking why is she intentially trying be noticed by sister. Visit to Homaspernia shows that, despite all that happened, Kendra is not just tormenting Maryen, but willing to help her by showing other side of those who her sister is trying to protect, to show that not everyone in this world are kind, mindful of their weakness and deserve to be protected.
MC worked perfectly to that btw. While Maryen used to save people from neutral towns, it wasn't like she loved each one of them. No, she did that because it was a right thing to do. But now, when there is someone she truly cares about and wants to protect not because of righteousness, but her own feelings, all these high moral values are going to get a rough reality check, which include resorting to violence, because sometimes that's the only language people will understand.
Depending on player's actions she will choose between trying to keep her status of a "Guardian of Heavens kingdom" (which means much more than just a protectress for her) and never follow the path of violence, with the help of MC of course, or satisfy her "dark" desires but still keep the balance without leaving her old life or becoming Kendra 2.0, also with the help of MC.
That is how I see it at least. I may be right, you may be right, or the story writer may simply turn Maryen into an absolute evil, we will find that out with future updates.