Yea, I don't know how it was at first, but right now it's like this.I'm not going to comment on how different Carla's personality is in the harem version and the NTR version, since I haven't played the second one (and I won't).
Personally I would prefer that Carla and Paul had a normal relationship (that it implies that they have had sex with each other doesn't bother me), and that, after discovering that he is cheating on her (again), she started a relationship with the MC. The father of the child isn't particularly relevant to me with these premises (the child could be Paul's, or the MC's, although certainly, if we think about what most fans of the harem genre like, the best option is that she is pregnant by the MC)
Well, you are certainly somewhat right. After replaying the game, I think Carla's personality and behaviur is better than my first impression (in fact, a great improvement for the original), so I guess I'll give her a chance before deciding if I'll definitely want her, or not, in the harem.
The end of chapter 2, when she calls to congratulate you on the new job, there is the flashback where they talk about their past.
She came over to his place. She wants sex. Paul comes knocking. You can ignore and fuck her and cuck Paul as he sees you banging her from the window. Creampie. She lies on top of MC as he hugs her and you can see her cry silently, showing us her conflicted emotional state and the peace she feels at that moment despite her questionable choices. Back to present time. Phone call confirms the baby is MC's. Then they talk about how her relationship with Paul is basically a facade and doesn't include sex. MC says the child is his and she loves him, so why not become his woman? She says she is afraid of getting too invested - too involved - because she actually loves the MC unlike Paul. She is afraid of getting hurt. A bit contrived, yes, but she never came off as the most straightforward or stable person. I personally like rocky relationships like this.
If you opened the door for Paul when he came knocking, the whole thing changes. She taunts MC, telling him that she will show him how a real man fucks her. Paul creampies her. Then she taunts MC to come get some sloppy seconds. MC is too infatuated with her to do anything but comply. Back to the phone call in the present. She doesn't know who the father is. She tells him that the sex that night was the best she ever experienced. She taunts him a bit and expresses her desire to cuck him again. There is just enough believability in the way this whole thing is handled to make this kinda interesting even though I don't typically like netorase stuff like this with a willing cuck. The MC's infatuation with her is something she holds over him.
She becomes 2 entirely different character based on the route. Make no mistake, she is definitely not a pure or good woman in either route. It certainly won't appeal to the pure harem enthusiasts, and that's fine. Not every game, or every harem tag, needs to be written for those specific harem fans. This game already has 169 patrons and making $622 at the time of this post, and the game is fucking new. It's been out for like 3 months. That's gotta be in the top 1% growth rate for porn games, so it's certainly doing something right.