I think there will be a parallel between Casper and Surya that will lead to an arc of redemption for Casper and one of decline for Callista.
Primarily, the incident was not a minor matter, and depending on each character's perspective, it can be interpreted differently.
I'm sure Surya will project his hatred onto Casper, because not only was he burned like him, but on the exact same side of his face. He'll surely try to constantly instill the same hatred he feels for Edina in Casper so that Casper develops a hatred for Key. But I believe this will have rather the opposite effect.
To continue, I want to share one of my favorite threads from this series: https://x.com/mochikattt_/status/2075303543662096644?s=46
As noted there, I believe Casper is in a state of denial regarding his mistakes in the series. He knows he's wrong but refuses to accept it, and I feel this incident will be the catalyst that helps him improve.
He knows that Surya ordered them to kill Key and is fully aware of what he told Key about his mother. I feel that when he has a moment to reflect, he will realize what he said and feel terribly guilty, since it is known from a comment by someone who worked on the series that the twins' mother is also dead. It would also be good if his father were the one to talk some sense into him. There's no way the man won't give him the scolding of his life if he finds out what he said about Key’s mother.
This, combined with Surya's attempts to make him hate Key and the fact that Surya is most responsible (along with Casper) for the burn, will make him realize his mistake and reconsider. My biggest bet is that he'll give up being a Bard.
Now, with Callista, she's the one of the twins with the more violent and aggressive tendencies. She knows what Surya asked for, but we still don't know if she also knew what Casper was going to say about the mother. From her perspective, it's likely she only saw Key explode out of nowhere and burn her brother's face. I feel she would be much more susceptible to the hatred Surya tries to incite than Casper, and that if Surya realizes he can't manipulate Casper, he'll try to manipulate Callista.
If what I theorize comes true, then Casper will leave the Bards and defend Key and the Knights, paralleling Surya, and this action would create a rift between Casper and Callista, causing the latter to fall into decadence at Surya's hands.
What do you think?