SPOILER
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so the way the novel describes it, her power of mind control works more like suggestions based on their deep dark feelings. so like with Derrick, she gets into his mind and finds the guilt he has towards taking his little sister out and letting go of her hand, which led to her kidnapping and disappearance. she uses that to plant suggestions like how pitiful yvonne is and how he needs to help her be happy.
With Eclis, she works off his obsessive and yandere-ish desire for Penelope, and plants suggestions like "if you rebel and take your rightful place, Penelope will be yours". And so by the time Penelope runs into him again, he's not really a mindless doll manipulated by Yvonne anymore, he's motivated by those obsessive feelings Yvonne brought out.
as far as frustration, its gonna get worse before it gets better but ohhhh does it get so satisfying