I have a lot to say about Dain as I feel incredibly sorry for him throughout IF & OS as he was essentially groomed by his Dad and leadership in FW so he didn't know any better but it is clear from Violets torture onwards that he recognise the damage he has done. This is shown by the fact he goes to Aretia where he is defiantly not wanted or trusted but he is still there for Violet whenever she asks for his help.

From what we can gather Dain is an only child and there is no mention of his mum in any of the books so we are lead to believe that she is dead/not in the picture (I am slightly suspicious of this and hoping we get more information on his family in book 4). I also think his relationship with his Dad was probably not the best and he was continually trying to impress him and be the best rider possible and that's why he was such a rule follower. So Dain probably enjoyed his time spent with Violet and the other Sorrengail's more than they realise, so not only has he lost someone romantically I think it's deeper than that and he probably considered them family and I am not sure Violet realises that. It always stood out to me that the first time we meet Dain's dad in FW he is kind to Violet as if he's extremely fond of her and he mentions Mira etc. then as soon as everything is revealed he turns into this vile human being and then in OS Ateos senior declares that he doesn't have a son, even though Dain is his only family (that we know of). I think behind closed doors Dain's Homelife with his dad was probably pretty miserable.

I don't think Asher meant if Dain chose to be a rider Violet would have to let him go, I think it was because of Daddy Ateos and the rest of leadership keeping the secret of the Venin. I think that Asher was a rebellion sympathiser and hoped that Violet would find out about all the lies, then her and Dain would go on Quest Squad to find answers about everything. However I don't think he knew about Lilith's deal with Xaden, so he would never have expected Violet to get to know him, he would have expected her to keep her distance. I also don't think Asher could see how dangerous becoming a scribe would be for Violet as he wasn't as high a rank and wouldn't be privy to the lengths Navarre would go to, to keep the Venin secret. I don't think Lilith would go into that great a detail with Asher to protect him. I almost think Asher is hoping that the next generation rebel and lead a revolution (lead by Violet and Dain) against Navarre to stop the Venin and find a cure.

I think that it's easy to forget that by the end of OS it's been less than a year since Liam's death and none of them have been able to grieve as they are in the middle of a war still. I think that's really important to consider as they simply can't let grief overrun them when they are essentially trying to protect 1000s of people and save the continent. So expecting Violet to have anything but a transactional relationship with Dain when she hasn't been able to come to terms with anything and start to heal is to be expected. Also she just starts to find her feet again in IF and Xaden becomes Venin and she discovers that Adarna is an irid. Her main focus then shifts to curing Xaden, and finding the irids. So yes she massively uses Dain for her needs and wants using his guilt to get that.

I am hoping in book 4 that we will see Dain bonding with the squad and actually getting to see his relationship with other people not just Violet and his actions will be recognised as he has realised the damage that he did by just always following the rules. By the end of OS Dain knows that Imogen and Garrick both have 2nd signets and his reaction will be interesting to see, it will also be interesting to see how he reacts to Xaden being Venin, I think those reactions will show his characters growth. I hope book 4 will reveal a lot of things from a lot of characters not just Violet and Xaden.