Idk ab when he fell in love with her but he got the bird during the hunting competition but he got it because she asked for it before the mc inhabited her body
He likely has feelings even before the main story starts going back to their childhood. Early on it’s shown he doesn’t like her to call him brother, and doesn’t accept her as a family member.
Pen thinks it’s because Derrick views her as beneath being part of the dukes family but now knowing he has a secret crush it’s likely he did this because he never could reconcile her being family with how hot and bothered she made him.
When did Derrick actually fall in love with Penelope? Was it when our MC inhabited her body or was it at some point before? I ask this because he owns that bird with the same hair color as hers. And it seems he has been taking care of it for some time. So I trying to piece together when because it also seems he had been harboring these feelings for a while as well. For them to grow so strong to the point where he not only pushes her away but willingly chooses to see her as bad as a justification for it, but I also believe that’s his way of taking out his frustration for his forbidden feelings he for her too.