Mearl is simply too cute...after being domesticated that is cause prior to that she was a psychopathic murderer. That probably hasn't changed but her domesticated cutesy side is simply too precious to think about anything else.
It's def nice seeing how each member of his harem has changed. In her case, when she went to watch Mikagami fight Alice, she casually choked out a dude and tossed him away like trash just to get a seat. Didn't even acknowledge his existence beyond that, so she's not all that much different overall. But she's a lot more cutesy and mellow when around Mikagami, given that is who she is smitten with. It's the same deal with Mary and Alice too.
Going off what Zulu said after Mikagami beat Mary, she likely wanted a partner who was stronger than her and could be the dominant one of the relationship to make her submit. We haven't seen much of Mary's old personality given she chooses not to watch Mikagami's fights from the stands, and chooses to remain waiting for him in their room. Meaning, she hasn't been shown interacting with anyone that isn't Mikagami or the other harem members yet.
In Alice's case, she's a misandrist with Mikagami being the exception, which even Zayd realized when she asked her about Alice helping Mikagami and Zayd saw her blushing. Alice told Zayd she was in a restricted area only for slaves, but then taunted her by saying if she wants to be part of Mikagami's harem, she'll take Zayd to his room lol. Even reaffirmed that she likes being on the winning side and stirring shit up, then noted how that's basically the reason for why she joined Zayd in the first place.
Fine will be interesting to see given her sadistic personality was just a coverup. Her real personality is nothing like that.
