It was Diana who tied up Perla, Kira, and Brenda when one of Diana's men arrived with the three of them. In the next scene, you see how Kira and Perla are tied up on the boat and then the MC arrives to rescue them before Diana kills them. Play that part and you'll see that I'm right. Besides, I just finished the game and I remember it, and even if I hadn't played the game in months, I would still remember it. Play that part and you'll see what I mean. And yes, Brenda told them things she would do to Kira and then Perla, because she thought they were both virgins, but that was when she was fighting with Kira, and then Perla arrives to rescue her sister, putting her sword to Brenda's neck, before the three of them hear Diana's fleet approaching the bay. Then what I told you happens; it was during the island arc, where it shows how Brenda tries to redeem herself with Kira when they are free or doing something on Diana's ship. That's why I said Brenda deserved a second chance, and if the MC shows her love and affection like he does the other girls, she could have been in the protagonist's harem, as well as being a good ally. The protagonist will need all the help he can get when he faces Baltazar and others, so what could be better than having three dangerous girls helping him? I say this because we already saw how Dalen, Borri, and the girls in his harem (Eliza, Alice, Luna, Silfide) rescue the protagonist from the dungeons, rescuing the princess from Tarón and Alaric.