Possessed Lillie is a result of Lillie herself becoming corrupted and taken over by a Nihilego.  The Nihilego now sits firmly on Lillie's head and uses her body as if it was its own.  Upon her corruption, Lillie's appearance has been altered.  Her normally white dress now fades to a deep gray the farther down on her body it is, her eyes are now a lifeless beige/gray, and her hair is now mostly an almost black color.  She answers to both and calls herself of the names Lillie and Nihilego.