Its a long story, i haven't read it completely as well but basically Claude will remember her back after Lucas come back with his full mana power (he went away to take back his power) it was hinted that Lucas will be Athanasia's love interest and Izekiel will yield back. There's a whole lot of context to fill in but thats the pretty summary. There will be lots of angst before we reach that point, but Claude basically will remember and know that he killed Athy in the alternate reality, and also will know that this Athanasia is not the same Athanasia he killed, nevertheless he felt really horrible (because in this current timeline, he really tried to kill Athanasia again) basically we will know lots of alternate version of the novel "Lovely Princess"
So when you say lots of alternative versions of the novel, do you mean that there are more lives of Athanasia than just the two we know about or do you mean that we'll see other points of view from Athanasia's first life? Neither option really sits well with me, though. Sorry! Although, I'm glad to see Claude regrets but most especially I'm glad to see he realizes that Athanasia, now, is different from Athanasia, before.
I think its the most obvious one that its not even a spoilers anymore. If you really read the part where Claude shares his memories to Athy, he begged Diana to choose him instead of the baby. He wants Diana to live and be with him, but his girl choose to save the baby instead. It was the prompt of his hatred towards his own child, he felt Athanasia took Diana away from him. Its an easy answer don't you think ?
Can someone who's read the novel, summarize and tell me the whole story? Cuz if I know the ending only then will I be able to tolerate all the drama and angst