In season one, Vincent and Tess aren't central enough to be heroes. Neither is William. If not being central is not a dealbreaker, then I might go with someone who never shows up in present time, Rupert Hawke, but that's not really correct either.
If "hero" means "focal character" or "protagonist" or something like that, we have four, not one: Diane, Kaab, Rafe, and Micah.
Micah is everyone's darling, but I'm not sure she's the hero in any sense other than "really important pov character". Her plot arc is that she is shown a wider world which, by and large, welcomes her, and her family, as represented by Cousin Reuben, supports her all the way. That's nice, but there's no tension here. This is the beginning of an arc.
I mean, yes, I'm on Team Micah -- if anything bad happens to her, there will be a lot of very annoyed fans ready to avenge her. Yes, she goes through angst and has a realization about lying to protect loved ones, sure. But, she's almost a mcguffin.
Kaab and Diane are dealing with the past and growing into their roles. They're magnificent. I love them. And... if a hero is someone who's on the side of good and right, they're not heroes. They're villains.
This is why I'm not sure how to answer the question because I don't think of them as either. I think of them as fascinating people. I could say I don't root for Diane, but that wouldn't be true. As this is fiction, I am rooting for her to deal with the situation from 17 years ago, both then and now, with utter ruthlessness. She is trying to be an asset to her husband, and I cheer that on, too. She's not really wrong about Rafe most of the time -- he wants to be in the center of power to get his lover and his school, but he can't be bothered learning even a veneer of manners.
There's only one thing that makes me look at the duchess coldly and say that she's a villain: William. Her business from 17 years ago involved a character who's almost a caricature. I don't care about that character. Ben is more than a caricature, but... honestly, not enough more for me to care. I care about Tess, but not Ben. Part of that is because, before I move Diane firmly into the villain camp, he's clearly blackmailing her. He's walked into something dangerous, doing something to provoke his death.
I first met Diane in _Swordspoint_, and her husband (confusingly referred to as Charles) does not appear in that book. All I knew was that he had gone mad. Perhaps he was abusive. Perhaps he was a horrible person. This would have fit some of what I was seeing in the City. But, certainly, he was an absence.
In Tremontaine, William is a person. He's a nice person, someone who cares deeply about academic matters, and someone smart enough to know when a prospective lover is faking affection. He's not perfect. He probably pulled strings to get his lover through examinations. He lies to his wife. And, when he yells at her, he is throwing in her face all she has done for him.
Yet, he has no idea what she has done for him, for she never allowed him to know. He has no idea that she sees through his lies, for she is determined that he should not. He's not perfect. But, he's a relatively decent human being, not a blackmailer, not a caricature. And when Diane destroys him, I care.
Kaab is a hero by her people's standards, or at least, a useful agent and a beloved member of the family. She is growing into herself, much as Diane did. But, at this point, Diane is acting for herself. Kaab is acting for her family. She tries to be good to those she loves outside the family, and, in season one, she does try to move closer to what Tess wants in a lover.
But, she knows what Diane is doing to William and keeps that secret, knowing it will hurt her friend Rafe. She can't not do this, no matter what Rafe promises, for William sane is a threat to her family. This is not a matter of social position, but of life and death, not just for her, but for her family, and perhaps for her country as well.
I sympathize with her, and I love watching everything she does, even when she is being foolish, but I don't think of her as a hero.
Rafe is wrapped up in himself, isn't he? It's all about his school, his exams, his feelings, isn't it? And he can't even be selfish effectively.
Yet, like William and Micah, and unlike Diane, he is interested in something bigger than himself, in knowledge and what it means, and, like Kaab, he is interested in a larger community, in all those who would learn. That is admirable. And he is quick to grasp what Micah's calculations might mean for his City, and for William.
And, Rafe is willing to give up all of that for William. It's not a possible deal at that point, but it's a deal he'd make if he could. He's also the man who grew up enough to send William back to his wife, however bitterly ironic that was. So, if anyone's "the hero" of season one, I'd say it's him.
But mostly? It's not about heroes. It's about interesting people whom I care about enough to watch and read about week after week.
Season Two? I expect more of the same, but I could be surprised.
Diane might come to care about the City as much as about herself. I've not seen it yet, but it's possible.
Kaab is being pulled in several directions, but right now, her family pulls the strongest. This makes her terrible, but it also makes her wonderful.
Rafe is in the same position as at the end, giving up so much for the chance of finding William. What I know and what I don't know are maddening as I try to figure out which way he'll move and what it will cost him.
Micah has not yet done anything, apart from math. I hope she gets to be more than a mcguffin.
Tess and Vincent are doing more. I'm rather annoyted at Vincent, as he's decided that Kaab can protect Tess as well as he can, and I see no indication that this is the case. Maybe he's distracted by a new arrival in the city. Certainly, those of us who've read _Swordspoint_ are watching him very carefully. As for Tess, she's starting to lose patience with Kaab, and I don't know where that leads. I like Tess, and I don't like the position she's in, but right now, she's an interesting person, not a hero.
Arthur, Ambassador Reza, and Esha are also interesting, but I don't yet know enough about them. I like the Duchess of Hartsholt, but I doubt she's going to become a protagnoist or hero. And William? Once more, Duke Tremontaine is an absence.
I don't know if there are any heroes this season, but I don't care. It's an interesting story full of interesting people.
That said, I do have my eye on a certain member of the Dragon Chancellor's staff and on someone interested in impressing him. And, the things that annoy me about Kaab do not bother me about Saabim or Chuleb. They're not the glamourous protagonists, but they're fighting for their family, their people, and their homeland, and they have no reason to be loyal to Rafe Fenton. Tell the story from the Kinwiinik point of view only, and everything changes.
But, that's the point, isn't it? We're not getting one side. We're not even getting two basic points of view, with a spice of a few others, as in _Swordspoint_ or _Privilege of the Sword_. We're getting something more akin to _The Fall of the Kings_, a panorama of the City and her people, with a loving eye, but not a blind one.
Perhaps that's it. Perhaps the City is the hero of the story.