NTR is just some weird Japanese acronym that everyone interprets differently.
In context of the game, I think Shani is *not* the MC's property for two reasons:
1) She is the 2nd MC so the player can choose what she does (within limits obviously)
2) Shani was raised to become a stalker. Ain was raised to be a wife / pet / bedwarmer. Ain is clearly the Zaton's property, either to use or to give away, but Shani isn't.
As you can clearly witness during Ain's personal time with Stalker, she has her own free will, and Stalker does not interfere with it. Just because Ain
have decided to devote herself to housewife lifetime on her own free will, does not mean she can be considered as a property. Not more than Shani. She just decided she will be more secure, more useful and more loved in the position she is now - Stalker's concubine. And nobody can say she is playing her role half-assed or not sincere. She is a true f*cking material and she is proud of it (like, why not? She has all the right assets), why would she be like Shani or Queen? She is herself and we love her for that.
So, it's like your relationship with a loving wife. Do you own her? Yes. Does she own you? Yes as well. Since you care about her and she cares about you. It's not a one-way relation.
So Ain's devotion to Stalker
is her own free will and decision, not a result of some kind of deception or corruption. That's why this relation is so lovable - she loves to be owned by him, and he loves to own her. Roleplay kind of relation? Sure, seems like it. Master-slave relation? No, because love feelings are mutual.
Same goes to Shani.
She gives herself to Stalker for his char/charisma, not because she is his property. That's why, being superficially linear, girls' relations with MС are not that trivial when it comes to detail, that's why they are much more lovable by players.
Even if they share the same models you've seen in other games
I agree, Ain may not seem like a very interesting character when it comes to the story advance, but 1) that is only for now, nobody knows how she will affect the main plot in the future 2) she is curvy, f*ckable, available and willing, you cannot lose with this combination.
We need more story to know more, but interactions with Ain are the main part which gives this game this kind of an alluring vibe. Whether you like it this way or not. )