I dont see where you people get these opinions. Obviously you are free to have them but I completely disagree. Luna is perfect for NTR. She is loyal to Leo to a fault which makes her perfect for NTR. Her devotion is so strong that its her weakness. All you have to do is use that weakness against her and its corruption time baby. Luna practically looks like she is a Succubus Army character just with her design alone.
Leo is focused on trying to learn as much about the treasure as he can while trying to be defend the Mansion and find the traitor all at the same time and he doesnt focus on what is happening with Luna. And once Luna gets corrupted she becomes so desperate to save Leo and due to her blind faith in his character that she doesnt realize Kurone is starting to sink her claws in him. Since nothing else matters to her but saving him trivial things like doing something lewd (which she does on assassinations anyway) become a quick and easy way to reach that goal and not such a big deal to her in the grand scheme of things. She believes she could never lose to a low class devil and that ends up being her downfall. He blind faith and obsession with Leo caused her to underestimate her enemy.
And since the game is called Mirage's Defection it was most likely intended for her to betray Leo and the Human side. The Dev even said on Ci-en that he used to have a route where Luna becomes corrupted by Iviel and then Sadi steals her from Iviel. She was always meant to defect to the evil side. Corruption and NTR are the genre's the dev enjoys making so why wouldn't the main heroine be used for those things?
The problem is with what your saying. Eviru is a terrible match up for that type of story and character. He's too stupid. Too weak. Too...Everything to acutally accomplish what he does beacuse he's littearly not smart enouph.
The problem is not Luna herself. Yes she works for NTR. But the SPECIFIC type of NTR that would be needed for her is a master manipulator type that uses her devotion against her and sets up scenrios where Leo hurts her or snubs her to let him sink his claws in.
The game didn't do this. Which is the problem.
Also no. The title is just a name. The 'Canon' ending is actually the pure love one where Leo and Luna stay together and win. So she was never intended to defect/betray humanity. But i see where you are coming from.
Luna is simply used improperly in this game. And the game itself even ackndowlges that the NTR littearly shouldn't of worked. At all. In the NTR ending.