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Angie Sirius (アンジー・シリウス) is a Strategic-Class Magician of the USNA and is one of the Thirteen Apostles. Out of all of the Thirteen Apostles, Sirius is the only one who's managed to keep a modicum of secrecy about their identity, to be precise only a nickname and code name are known, not even their real face. 
The most elite of the USNA Military's Magician Unit "Stars", the Head-Captain is code named "Sirius" is the magician who possesses the strongest magical power.  It's known as a rumor that those bestowed with the title of "Sirius" are front-line fighters.