The way I understand it, SIN's scheme works like that:
Zara's marks provide information about the location of the target (probably among other things, but we don't know about them yet). They work no matter if she personally places the mark or if someone else paints it.
Once someone marks himself, Zara can pinpoint the location and instruct the Hexenringe to teleport him, probably to SIN's headquarters.
There, the prospect swears an oath to Charlie. The oath probably has to do with keeping some things secret, like the names of SIN's management, which is why Nyx's prisoners died when they tried to give information.
We don't have any idea about the extend of Zara's power but her marks currently don't seem to do anything more than track the marked person.
Some possibilities is that she uses the marks as
Homing beacons for projectile based attacks.
Getting knowledge about the marked target's ntent so she can fight better against them. Better targeting, better dodging/damage mitigation, some other unique insight.
Use the mark to influence behavior.
She could also be a communication specialist, allowing the marked people to create links and communicate telepathically at distance, perhaps even sharing knowledge and unique insight non verbally..
Zara's mark most likely has a sort of eavesdropping mechanic to it, yeah. When the MC talks with the HERO Captains after the Nico and Hex fight, you gain a point with Ella, if you don't tell everything. The MC was still marked by Zara during that entire thing.
I definitely only just hate her because she's an asshole, she's a well written character though. She's the type of person I'd hear speak in real life once, and I'd nope out of there and never interact with them again. Trashtalking the mc isn't necessarily my problem with her, it's just her overall personality that I have a problem with, of course the mc got trashtalked because of said personality but that's more incidental than anything else. Love her design visually though and am looking forward to when we can take her down a peg or two, same as with Ella.
I mean, that's what I said, no?
"Most people just hate that she's vocally prejudice against people, can be extremely selfish and doesn't care about the MC's status as MC".
You fall squarely in that camp, no? You might not hit that last mark for the MC thing but you do dislike that she's an asshole, overall. Which is the problem most people generally have with her. Even though her asshole tendencies aren't targeted towards them, for majority of her appearance. I don't care that she's an asshole to everyone, personally. My MC is one as well. So is Ella and several other characters in game.
Nico legit only did one harmful thing to the MC and that's leaving him hanging during the Valravn fight. Something she made up for by being on time and helping with the Cole thing. As well as actually helping us all escape Hex collapsing dimensional space. But no, she's an asshole to the MC once and to everyone else, so that's all negated, apparently. Ella telling us that she's manipulating our lives is cool for most people(not you specifically) but Nico being an asshole? That's the thing that's too far, I guess.
As for the bottom half, we're on the same page. I too look forward to bullying Nico, sexually. I don't care to knock Ella down though. I rather surpass her, so I don't have to follow her plans, and then ignore her existence completely. I feel like that would hurt her more than any other thing we could inflict on her.
Overall, I get your pov on the subject.