I have rough idea about this after reading very rough translations. So there might be some facts left out.
SPOILER
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First of all, one thing that must be remembered that Penelope has transmigrated into this game world when the game world is set into the Hard Mode. Meaning that, she is continuing her survival in the Hard Mode and this Hard Mode entitles that it doesn't matter how much Penelope achieves favorability with the capture targets because if she slips up even just a little bit, all these favorability will instantly decrease depending on the degree of annoyance the capture targets feel at that moment. This means that her life and death depends on the fickle emotions of the capture targets which is really shitty as fuck. (╯°Д °)╯╧╧
Anyway keeping that in mind, Penelope actually cannot trust in the seemingly rising favorability of Ecklis or any other for the matter. Because just as it seems that Ecklis' favorability is rising quickly, it is equally easy for that favorability to fall down and even down to negative. Only when the favorability is 100% can Penelope hope that regardless of her behaviour, the favorability will not fall which will in return ensure her survival. Anyway this is the premise of her survival. And the reason for Penelope's unenthusiastic response towards Ecklis favorability is that in this Hard Mode the capture targets favorability towards Penelope is fickle as fuck. There is no sincerity behind it as you can see in this chapter that in Normal Mode Ecklis pledged his love for the Heroin, but in this Hard Mode Ecklis only pledged his loyalty despite having a relatively high amount of favorability. Thus Penelope's only goal is to find the most likely easier capture target among all the capture targets with whom she can increase the favorability quickly so that she can avoid her death. She wants to do this especially before the Heroin enters the picture because once the Heroin enters the Hard Mode will revert into Normal Mode and as you can guess, in the Normal Mode the capture targets will fall for the Heroin quicker than you can say poop ( ̄へ ̄)
And as for the reason Ecklis betrays Penelope: Ecklis didn't love Penelope. He had an obsession towards Penelope and wanted Penelope to only depend on him. Thus he brought back the Antagonist (Here I think the translator meant the Heroin) in the Duke's mansion so that Penelope will loose her identity as the Duke's adopted daughter and get kicked outta there ya know. I mean the guy lost his counrty, family, identity, freedom, dignity all in one scope. So I guess it's quite normal for him to have more than a few screws loose inside his head (〜 ̄△ ̄)〜
Ecklis is like a hidden antagonist here besides the main antagonist.
I was so upset at Eclis' ending that I didn't even stop for a minute and think that he might have became like that because of what happened to him. Thank you for adding that, now it make more sense that he ends up obsessing about Penelope because he literally lost everything and was left with nothing. I guess Penelope became his everything and didn't want to give her any chances of throwing him away and leaving him with nothing once again.
Thanks for the spoiler!!! (づ ̄ ³ ̄)づ
*Spoilers alert*
Ok this story is really confusing me, i dont understand the relationship between Penelope and Eclis maybe its cause I've read some spoilers and i saw that Eclis will betray Penelope ???? Isn't Penelope trying to seduce Eclis?