[SPOILER]
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I feel like they're only changing Helios so we don't hate him too much at the end of the webtoon, since in the novel he had a somewhat happy ending that didn't feel deserved by many causes of his actions/feelings toward Hes. In the same way, they're making Diana more annoying in the comic than she was in the novel (honestly, she's a pain wherever but at least she was a little bit less dumb in the novel), probably cause some people still pitied her after she got her bad ending. Cael already cares for Hes more than he does for Diana, it was hinted at a few times. The thing is he just wants to be happy with Hes and doesn't care for revenge so he isn't as against Diana as Hes is. Diana is so "meh" to Cael now he doesn't care what happens to her and just talks to her out of politeness because she's an old friend now royalty who married his old best friend.
Can you tell me what happened in the novel?
I hope Cael don't change and care only about Hestia, coz in the novel it was a different scene in the library, I hope next chapter he ignores that ugly bitch and to not have any feelings left for her