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Eclise is an obsessive (kinda yandere) character. He wanted Penelope all for himself. He once told her that they should run away (from the ducal house) which she refused cause she could not take risks. After being refused, he showed up with the actual daughter because that would mean Penelope will have no one but himself (Eclise) to depend on as he is aware of her standing as fake princess. But he did not want Penny to die so he protected her. So, i am not sure whether to brand him as a traitor in an overall sense or not though he did betray Penelope's feelings and interpreting it to his own convenience.
GRAND SPOILERS ( for the impatient ones)
proceed at your own risk (don't blame me if you see more than you can handle)
SO, After Eclise reaches a high affinity (the one above their head), around 70, then instead of the numbers there is a color for all the leads. Penelope needs to either pay with money or fame points. The meaning of the color is in the link below which is like really important.
https://m.blog.naver.com/akazihaeh/221753431857 (This site will literally give you almost everything you are curious about so yeah, if you are a spoil masochist give it a go)
And you know what, Eclise brings back Yvonne, the actual daughter of the duke. He BETRAYED penelope. When in actual Vinter should have been the one then he brings her back 5 days before the deadline of hard mode making penny go crazy.
And to those who were curious about this, Eclise does die.