actually in the novel he appeared when athy left claude's mansion after his memory loss, she went to an all boys school out of curiousity and thats how athy and cabel met. He was supposed to be the 3rd love interest of athy and he didnt actually like jennette, he thought he liked her or sum bcoz of the effect of jen's black magic but the author made so many changes here, they went with the jennette route instead of cabel route as athy went to jennette after leaving the mansion so idk if cabel's gonna appear here or not
You probably read a fanfiction or something. Nowadays there are fakes everywhere, especially on wattpad. In the novel Cabel was never interested in Jeannette. He was brainwashed by Jeannette's black magic so he "fell in love" with her and forgot Athy for a bit, then recovered once the black magic was gone and started to like Athy again.
In the manhwa he will not appear tho as Spoon said, since it will be more family oriented. We will probably have little to no romance even between Athy and Lucas.
I know a character from the other novel "beware the brothers" makes an appearance Cabel Ernst and I think he becomes the love Interest of Jennette. And Lucas and athy grow closer.