Toussaint is the servent of Jean Valjean, Cosette, and eventually Gavroche, Pressoir and Jurges while living in Paris. She is seen as a very motherly figure towards Cosette, and the two forge a close bond while living together.
Toussaint apparently met Valjean by chance when praying at a church one day, sometime after her son and daughter-in-law died from an epidemic. Not long after renting their apartment in their second residence in Paris, as well as having a benevolent personality, Valjean hired her, likely after hearing her backstory and lack of food as a result of the happenings during and after the epidemic.
She appears in every episode of the series from her debut in episode 23 onwards.