Basically he changed the whole plot. he made decisions that will benefit him that does not coincide with Penelope's plans. It's just that his love was concluded as Love to death and he have to suffer from it. I cried reading the novel. Eclise tried really hard. I hated and loved him in the novel. I kept crying his end was inevitable but i feel like he deserved another ending. ╥﹏╥
Hmm, betrayal huh...if you really want to know, then
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Eclis knew that Penelope was lying about liking him from the start. At first, I think he planned to use her and escape, but then he became obsessed with her and decided that he wanted her to escape with him. But things were going good for her (relationship stats wise), so she wasn't in a hurry to run.
So Eclis decided to FORCE her out of the house by bringing the Real Daughter home...and Penelope dropped him like a bad habit, because that wasn't a harmless prank, but would actually kill her (which he did not know at the time--he just wanted her to run away with him.)
Add in that the Real Daughter had super brainwash powers and kept winding Eclis up, and he was already predisposed towards being a yandere (plus extra unbalanced because Penelope rejected him), and Eclis is a problem waiting to happen.
He's certainly tragic enough. Penelope feels bad for him, too. (Bad for leading him on, anyway.)
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Except for the fact that he brought home the Real Daughter (who is actually a dead body animated by a semi-divine being who has killed original!Penelope hundreds, if not thousands, of times, and literally orchestrated Armageddon every time--and plans to do the same thing again). Which Penelope figured out...oh, a month or so before Eclis brought Real Daughter home. So his "small" rebellion had earth-shattering consequences.
Real Daughter then manages to get Eclis to help her with Evil Plot in return for not killing Penelope (for the two hours it takes to kick off Armageddon *eyeroll*), and Eclis abducts Penelope, locks her up, and tries to kill the ML because he's a yandere.
eclis is so precious but it's sad that u will betray penelope later on : ((