During a trip to Paris aboard La Baleine, Amelia was stricken with a malady that warped her true name—becoming Eglantine, the Prison of Briars—and lashed out at Noé shortly after meeting him. Vanitas managed to cure Amelia's ailment with the Book of Vanitas, revealing her true name to be Florifel, She Who Guides Spring.

(Source: Pandora Hearts Wikia)