While we only see her in brief flashbacks, it's clear that Momma is the source of all that is good in Isabel—we're talking about her kindness, bravery, and loyalty to others. Apart from Momma's relationships with her children, we don't know much about her, except that she was enslaved from Jamaica and lost her husband when he was sold to a different family than she, Isabel, and Ruth were. Still, these sparse details speak volumes about the kind of woman she was and raised her daughter to be.
Above all others, one detail stands out that captures who she was:
She wouldn't let anyone hurt her children. (10.8)
To this end, as she copes with her situation, Momma's comfort and protection often guides Isabel as she overcomes difficult situations. For instance, Isabel sings a Jamaican lullaby to Ruth the night Madam hits her in the parlor, and responds to the sadness of spending Christmas alone by making bread pudding for a homeless Loyalist family.
In the midst of her fever following the branding, it is Momma who, in Isabel's mind, comes to her bedside to comfort her, and she even sees the fireworks that distract the British soldiers enabling her and Curzon to escape as Momma's protection. We can't say for sure, but it's likely that Isabel never would have made it on her own if Momma hadn't first been strong throughout her own life.