Countess Marie is apparently a genuine member of the aristocracy who ran away to join the circus; she claims to be distantly related to Queen Albia of England. She married the Circus Master and helps run the whole shebang.
A Lady of Many Skills
Marie is a slightly sparky chemist, and one of the main architects of the Circus's escape from Sturmhalten: she invented the hallucinogenic gas that caused the Circus members to appear to really be the Heterodyne Boys. As seen in the attached picture, she usually sports a pair of Sparky goggles on her forehead, but to date we haven't had an opportunity to see them in use.
She is also a highly adept and dangerous practitioner of frying-pan-fu --. Wooster is as well -- the next time we see him, Marie successfully taught him respect (fear?) as well.
Following the aforementioned battle of Sturmhalten, she decamps to England with the rest of the Circus; when Agatha finally catches up with them there, Marie has the running of the Circus's scientific fortune-telling tent previously manned by Olga and Agatha, under the (presumably) nom-de-thesp Madame Zulbravia. During the main performance of the troupe's new Agatha-centric play, she is assisting with costumes and wearing a completely different outfit and hairstyle; she looks so different, both from Madame Z and her appearance during her Continental days, it's not immediately clear that it is her.
Her depiction in the print novels hews fairly close to what we see in the comic. We do learn that she's also an expert at ruthlessly getting incapacitated (as in inebriated) Circus members up and moving when they are needed on stage, and that she usually avoids unseemly public displays of affection like holding hands.
Countess Marie appears in "The Works" where she is given the attributes Circus - Chemist - Chef. She is depicted as using a blue ladle to taste something cooking in a black pot.
(Note: the Chef does not have a card in The Works.)