Itsuwari no Bara wa Fukushuu ni Somaru

Cosette lived a poor life alongside her mother.
One day, she finds out she was born to a Count as his hidden child.
To get revenge on this nobleman who harmed her mother, Cosette disguises herself as a boy named "Gilbert" and declares she will become the heir to the Count's family, aiming to become the perfect noble child while hiding her gender.
But one day, at a ball, the gentleman Wolff discovers that she's a woman, pulling her into the nobles' waves of malice. Will Cosette be able to successfully exact her revenge?

(Source: Pocket Comics)