I've already written Mady's story in draft form and though I'm not sure how much of that will remain the same, her general arc and the events of her life are basically set.
Madeline is the Miles's second daughter. She's born in late 1794 on a plantation in Barbados. Her mother is Delphine (which is a French name--maybe mama was partially from St. Domingue, which later became Haiti?). She's a free black woman and though I haven't read of any specific laws at the time stating that white British men couldn't marry free black women (there weren't any such laws in England itself, ever), it couldn't have been easy for Delphine, Miles and their little family unit. Mady is very close to her older sister Alex. I'm not sure when they realized that they could not have been born to the same mother or, later, when they realize how different they are in society's eyes, but as they have a strong fabric between them, it's not anything that affects their relationship.
Madeline is shy but sociable. She's very ladylike and enjoys a pretty dress, but she's also quite inherently maternal. As they grow into adulthood, it is Mady who is always reminding the more intense Alex to eat properly, to do her hair up, and to dress for the occasion.
As children, Mady and Alex share a bedroom and they have secret worlds that they have built up between them. Mady is a voracious reader (she particularly enjoys Sense and Sensibility).
Then Miles dies, sending the household into mourning and chaos. When business and money troubles crop up, Mady tries to educate herself on the matter. She inherits some of Miles's self sufficiency in that way, but Mady is more traditional than her father. She has begun to think about marriage at this stage. She's an heiress; does she need to marry? Who will she marry? What kind of man will have her?
That's when she meets Henry, who is a half Indian, half English shipbuilder. And trouble and romance ensue. At the same time, Mady becomes involved in an emancipation society and helps found a ladies' emancipation society. She becomes more of a rebel as she grows older, but even then, she's quite subtle about it.