I think what keeps bringing me back to the high-cloud quintet is how they've all hurt each other so badly and yet none of them can truly be considered an antagonist. There's no world where any of them could realistically have chosen differently. Each one of them is both victim and perpetrator in one way or another and it makes for such a beautiful tragedy.
On the surface and I mean realllllly on the surface, Dan Feng is the antagonist. Because he's the one who made the choice that screwed up everything else. Except did he have a choice, really? He straight up said in 3.6 that knowing everything that came after, he still wouldn't choose differently. That's not him being dismissive, there was just no possibility of him letting Baiheng die while he might have had the ability to save her. He was so lonely before the quintet and no one ever taught him to let go because no one ever thought he might be human enough to struggle with it in the first place. It is literally his Aeon-designated nature to want the things he cares about, forever, and it's kind of impossible to treat Aeons as true antagonists because THEY, too, are slaves to THEIR nature. It would have been impossible on a philosophical level for Long to create something that knew how to cope with loss.
From a thematic standpoint, you could maybe argue that Jingliu is an antagonist. In a story about moving on, she's the one who clings to the past the hardest, who forces her ideals on the others. She wants to punish Dan Feng and Yingxing for refusing to let Baiheng die, but in doing so she's also refusing to let Baiheng die. And yet. When was she afforded any other opportunity? She was never in a position to empathise with them because between the Cangcheng disaster and dedicating her life to the sword, loss had become an inevitability to her. The Hunt was entirely her way of life; transgressions should be met with punishment, that was simply the way of the world for her. She had dedicated her life to vengeance long before the quintet was even formed. Coupled with the fact that she was already nearly mara-struck, she never truly had any other choice. In theory, maybe, she could have chosen forgiveness, but her past experiences had already removed that possibility.
I don't think anyone will argue for Blade being an antagonist but let's talk about him anyway: Yingxing was so desperate to prove his existence a meaningful one that I don't think he was ever particularly morally sound. And as a craftsman, he believed that everything could be fixed. He could never bear the idea of not taking action because he ended up being emotionally dependent on proving that being a short-life species, the thing that the Xianzhou treated him as lesser for, was actually what made him better than them. If he did nothing, treated immortality as a taboo without even trying to fix things, then he'd be just like a Xianzhou native. Are we noticing a pattern yet? The world had already shaped him into the person who'd try and help Dan Feng because he didn't feel like the Xianzhou was something worth obeying. And then, past the point of the Sedition, his actions are tainted by Jingliu's 'training' which she also had no choice in not only because she was mara-struck but because punishment and vengeance were all the world had taught her to depend on.
Jing Yuan is... interesting. He did have choices, and he made some pretty good ones. He was the only member of the quintet with healthy experiences to fall back on and it definitely shows. He did hurt the others in that he moved on, he dealt with Jingliu, he did his job as general. He says as much when he's borderline emotionally blackmailing Dan Heng to get him to help with the Arbor Crisis: he puts the Xianzhou before his feelings. Technically, yes, he had the choice, but he also had the wisdom to know that nothing good would come of intervening. It's not as if he did nothing- he fought to get Dan Heng released from the Shackling Prison, he let Blade go after the Arbor crisis- and some of it was due to his feelings, sure, but he's always acted in the interest of causing the minimum possible disturbance. The safest thing he could do was bear the burden on his own, bury his feelings, and try to get on with his life on his own, and that's not something he deserves to be faulted for.
And then finally we get to Dan Heng, who unsurprisingly inspired this post. How much easier could things have been for him if he had accepted from the start that he was Dan Feng, and that he'd hurt people but he'd done it because he was just so painfully human and he loved too much? If he'd recognised Blade from the start instead of constantly running from him? If he'd been able to share his past with the Express Crew from the beginning? We'll never know the answer because between the amnesia and the growing up in prison and the self-hatred inherited from Dan Feng, that was never a possibility.
There is just so much tragedy in how all of them are so helpless to their own natures and how technically everything was preventable and yet in reality all of it was completely inevitable. Nothing is fair and nothing can ever really be made right. They were all just people doing the best that they could with what they were given.