Elizalina

Elizalina is a magician, the twin sister of the Maiden of Versailles and the ruler of a country named after herself which has recently gained independence from Russia.

During World War III, she agrees to help Kamijou Touma and Lessar against God's Right Seat's Fiamma of the Right but only until they help her make Sasha Kreutzev leave her country as she doesn't want to be involved in the war between Academy City and Russia. However, Fiamma attacks her palace and proceeds to defeat Elizalina, her guards, Touma, Lessar,Vento of the Front and succeeds in capturing Sasha.

She is later heard by her sister and Princess Carissa, guiding them while they're dealing with rampant manifestations of Telesma.

(Source: Toaru Majutsu no Index Wiki)