Madame Desmana is/was a Parisian dealer in gray-market Mad Science items and information, operating from her own shop in the city's infamous Black Market. She had interactions with Ardsley Wooster when the latter individual was living in Paris.
Before dying, she had a copy of her personality grafted into a clank constructed by the Hurwood Brothers, Sparks who went on to run afoul of Simon Voltaire and Gil "Holzfeller". This clank continues to run her shop, and Agatha when our heroine comes to the Black Market seeking the whereabouts of Professor Dio Zardilev. Desmana is unable to help Agatha with this, but does serve as a broker selling the technical specs on Agatha's Wasp-Eater to any interested parties with the necessary cash. The clank is suffering "typical post-vital personality drift", which seems to produce hostility toward humans and/or feeling superior to them, but it seems to suppressing this and Hoffmann (and unnamed others) are "keeping an eye on it".
She has a standard policy of charging Martellus von Blitzengaard double-rates whenever he makes use of her services.
"Des" is a French combined preposition article "of the" (plural). "Mana" is a Polynesia term for supernatural power that has come to be used as standard game terminology for magical energy quantified. Hence Madame Desmana would be "my lady of the magic(s)."
Alternately, the Desman is a type of semiaquatic mammal from the mole family; it has two species, one with genus desmana.