Dain
I've seen people say that Dain gets treated badly so this is just me going over his relationships with the other characters and my thoughts on them.
Violet spends a lot of time believing he was in on what was waiting at Resson, that he was fully and knowingly a part of the plan to send them all to die and spiralling, wondering how many times he spyed on her and violated her privacy. She doesn't learn otherwise until later and it takes a while to let go of her anger, and unlearn what she thought was the truth about him. They talk and begin repairing not only that, but also the rift that formed between them during the first book. Dain recognizes the Quadrant did change him in the year the were apart and he didn't listen like he used to but Violet also starts seeing things from his perspective and where he was coming from.
At the point they're at in Onyx Storm, their relationship is still healing but Dain is commited to be there for Violet in a way he hadn't been during Fourth Wing, actually helping her, supporting her and trusting her skill. Violet is not just using Dain, he *wants* to help. He wants to put his skills to use, do his part.
They're not seen hanging out as much because, not only is their friendship still recovering, both Dain and Violet are extremely busy and no longer on the same squad. Dain has his Wingleader responsibilities that he takes very seriously and third years are also being called out to fill in at outposts more than they ever were before. Violet says at more than one point in the book that her time when she's not in class is a blur of research and training to the point where *everyone* barely sees her.
The teasing he gets isn't even that bad. Rhiannon calls him out once on taking himself too seriously on the turred in Iron Flame and reminds him that Xaden didn't act the way he was acting to get the respect he's aiming for and Ridoc makes the comment about Dain still turning out "halfway normal" with the upbringing he got and thinks he's not quite ready yet to handle the venin bombshell in a productive and helpful manner. Mira is a hardass, but when has that ever been news? But its not like he's getting jeered and bullied and iced out.
The greastest culprit is Xaden. Their dynamic is complicated. During Fourth Wing, Dain can't be trusted, especially not with his signet and his familial proximity with somebody with the power to really screw them over. He assumes that he's like the other navarrian loyalists we see and that assumption isn't exactly coming out of nowhere at that point. He also doesn't like the way he treats Violet. In his POVs he makes note of him holding her too tight right where the wound on her arm is, him fussing over her and smothering her, him not helping her get better at sparring the way a squad leader is supposed to and it pisses him off.
But at the same time, he's still jealous because he has a whole fifteen years worth of history with Violet that he was not a part of it. Yes his shift in behaviour at Narelle's was abrupt, but hearing they were so inseparable that her father had assumed she'd be with Dain touched a nerve. *Especially * since, at that point in time he thinks Dain still has feelings for Violet. He doesn't know he's began to move on, he wasn't present in the scene where he's hitting on a healer at the bar and the Slain hints hadn't started yet by that point in the book. Xaden doesn't feel worthy of Violet's love and he does carry insecurity about not being good enough for her and thinking she deserves better.
Xaden is still having trouble trusting Dain. He does blame himself for Liam's death, he says as much in Iron Flame, but Dain's part in it was still painful. He has full confidence in Violet's judgement to trust him, but he's not quite there yet himself, he's still prickly. There's just too much to get untagled all at once. But Xaden is showing progress slowly and he did save him during the battle.
I think Dain will be more involved in the inner circle now that he's about to be an Aretian Lieutenant instead of an overworked Wingleader and Violet as Duchess will need to keep her friends close more than ever.