A couple things.

First, a word on game mechanics.
Quote Originally Posted by Mightymosy View Post
What makes me think so? She is a shapeshifter character. Usually, when there are shapeshifters, they dominate the entire story. Everyone could be it. The whole story usually revolves around the shapeshifting ability and you are constantly wondering.
Especially if said shapeshifter is a a succubus, a demon from hell that probably has a lot more supernatural powers, which seduces men and then kills them. Usually when such a character appears in a story, she (or it) warps the entire thing around them.
Here's what you have to remember about the vanilla succubus as presented in the SRD: she's a glass cannon with a few decent abilities all of which rely on either surprise or subterfuge. As a CR7 with 6d8+6 hp, any halfway decently-built character I've ever seen could oneshot one of those on a non-crit. The less-well-built characters might take a whole round, and only someone who doesn't know what he's doing or isn't expecting to fight anything would get caught without protection from evil at that level. (please note that my perspective is that of a DDO player, and thus is admittedly skewed toward a very "high magic" type of setting). As such, a succubus can be a dangerous foe, but not to characters of the Order's level. Even back when she first appeared, Sabine always would have needed a conveniently tailored plotline to pose any kind of threat.

Now some more words on storytelling
Quote Originally Posted by Mightymosy View Post
The story isn't over, so of course I can't know about all of Sabine's motivations, but so far she seems such a natural character. The strip where she watches TV at the fiends' place is a super example. Inside her demonic skin there is a girl with "human" emotions that drive her
I see it as the other way round: layered on top of this demonic creature are the cultural expectations of a real-world, internet-enabled, English-speaking audience (that's us), being toyed with in numerous ways for the sake of humor. It's a similar narrative trick to the crystal ball wired to a TeeVo which subsequently turns out to have an "Eviscerate" button: take something alien, give it familiar qualities to make the audience think of it as familiar, then demonstrate it's still alien after all.