Let's start with blood character-relations for Lady Seymour. Lady Seymour is Master Lockton's aunt. Master Lockton and his wife are the two characters that take over ownership of Isabel and Ruth after Miss Mary Finch dies. Lady Seymour is also crazy rich.
The reader expects Lady Seymour to be an older version of Mr. and Mrs. Lockton (super-mean to slaves), but they, like Isabel, are pleasantly surprised. Lady Seymour treats Isabel kindly and with a great deal of respect (even though Isabel is a slave and the year is 1776). That's a really a progressive attitude for the times. Here is one of her thoughts:
"I find the buying and selling of children most repugnant"
At one point in the story, Isabel is branded on the cheek by the Locktons, and it is Lady Seymour who takes Isabel in while she heals up. Lady Seymour is one of the bright spots in the story. While most of the slave owning characters in the story treat Isabel and Ruth very poorly, Lady Seymour has a heart, and she genuinely cares for her fellow man ... regardless of race.
In Chapter 9 of Chains, Lady Seymour, Mr. Lockton's wealthy aunt, is introduced into the narrative. She is a foil character to Madam Lockton as she is kind and Madam is very cruel.
An extremely wealthy woman, Lady Seymour has very influential connections and is able to wield power over others. For instance, after Mr. Lockton is arrested in Chapter 11, she is able to obtain his release. In Chapter 24, Lady Seymour comes to the aid of Isabel after Madam has had the girl cruelly branded with an "I" for insolence. Lady Seymour sends Isabel to her home to serve because she has soldiers staying there, among them Hessians. There, too, Isabel can recover her health.
In Chapter 31 Lady Seymour's house catches on fire as fires burn throughout the city. Lady Seymour tries to salvage her most prized possessions in a box, but she drops the box and only those things that Isabel has stuffed into her pockets survive the fire. Once out of the burning home, Lady Seymour, who is helped by Isabel, collapses by a graveyard. Finally, they make their way to the Locktons' home.
The next day Isabel realizes that the kind woman has suffered a stroke. "The doctor said it was an apoplexy brought on by the fire." Lady Seymour recovers somewhat and thanks Isabel for the few things she saved in her pockets. In repayment, Lady Seymour has seamstresses make a warm winter skirt and a cloak for Isabel.
Later, Lady Seymour's condition worsens. However, she still has her mind, and she learns that Isabel is sneaking food to the prisoners. She warns Isabel not to let Madam Lockton learn what she does or she will be severely punished. When Madam does discover what Isabel is doing, Lady Seymour takes the blame, saying that feeding the prisoners was her idea.
In Chapter 40 Lady Seymour apologizes to Isabel for failing to protect her better from Madam's cruelty. She did try once to buy Isabel from the Locktons, but Madam refused her offer.
Not forgetting the kindness shown her, before Isabel quits the house of the Locktons and heads to New Jersey and freedom, she stops by the bedroom of Lady Seymour and rekindles her fire. "She wouldn't die of cold not on this night, not of my account." Isabel is grateful to her. She has done her many kindnesses, the last of which is to grant Isabel the money she sees Isabel take from her purse to aid her in the escape.