What was Esther's Hebrew name?

Answer: Hadassah

Mordecai had brought up Hadassah, that is, Esther. Abihail was the name of her father.

“And Mordecai had brought up Hadassah (that is, Esther), the daughter of his uncle, because she did not have a father or mother. The young woman was lovely in form and appearance, and when her father and mother had died, Mordecai had taken her as his own daughter.”— Esther 2:7 (BSB)