I will start as nice as I can be about Madam Lockton's characteristics. I would like to start there, because it will be a short list. She's not a nice lady.
First, Madam Lockton is wealthy. She has to be in order to afford multiple slaves. In fact, she's wealthy enough that she intentionally raises her bid for the girls so that Jenny can't buy them.
Second, Madam Lockton is a lady that stands firm for her beliefs. I disagree with most of her beliefs, but it is admirable how much punishment she is willing to take for her beliefs. For example, she objects to her husband's demand that they go into hiding after the plot to kill Washington went bust. She knows better than to disagree with her husband, because he's physically abusive to her; nevertheless, she still states her opinion and takes a beating for it.
Beyond the above two items though, I can't really find much more positive to say about her. She's incredibly cruel to the people that she is in charge of (owns). She savagely beats Ruth at one point and even goes so far as to have Isabel branded. Branded! Like an animal. She's just as violent tempered as her husband is, and she is quite selfish. I think Isabel's mother sums up Madam Lockton quite well with the following lines.
Everybody carried a little evil in them, Momma once told me. Madam Lockton had more than her share. The poison had eaten holes through her soul and made room for vermin to nest inside her.