In the first part of the series, I'd pick Mugino. She gets softer in New Testament, though. Soft enough to chat with Last Order on the streets, turn off her artificial vision because Last Order found it annoying and calmly reply and act like a normal person to her.
Levinia is an option... But I can't really view her seriously since she argues with someone like Fremea, thinks Santa Claus exists, sleeps with a nightlight on and such.
Hm... Kihara Amata is also quite ruthless, but he dies too soon and he quickly gets overshadowed by other, more powerful characters.
Maybe Marian, but while she is a sadist she also seems to have soft side to her, not attacking those who are uninvolved,chatting and caring for comrades, and such. I wouldn't say she's suitable.
I'd pick Othinus. she's a ruthless and cold-hearted person who will utilize any means necessary to achieve her goals, regardless of how much destruction and suffering they bring to others. She has no qualms about threatening or harming anyone who inconveniences her in the slightest way, be they an enemy or an ally. And she simply has the power that Kihara lacks to be on the spotlight.
Yep, Othinus it is.