
Sure, you can say that he's brainwashed after they met Yvonne, but his abuse of her started way before he got brainwashed. While the 2nd brother also did abuse her, but he decided to convey all of his anger, regret, thoughts, insecurity and so on to Penelope sincerely and he matures and takes responsibility for his mistakes. Derek never said a single word of apology at all. Even after the brainwashing was no longer effective.

Yeah, but his abuse of her started way before he was brainwashed. While the 2nd brother conveys his thoughts, anger, regret and feelings sincerely to Penelope, Derrick never said a single word of apology at all. No sincere apology for disregarding her when she was in the prison, no sincere apology for misunderstanding her during the poison case, none at all. And he also never sincerely apologized for his abuses. While his younger brother matures and takes responsibility for his own mistakes, Derrick doesn't. Out of all the five male leads, I hate him the most.
i thought her family had a chance of redemption like penelopes family but nope. theyre horrible people. anyways i need to prepare my heart for when Jin gets rejected, we all know its honna happen and its gonna be SAD