Descending Angel Enchel Lena

Reina Tomoe (the protagonist) is an ordinary girl who attends a very typical academy. Pure and an excellent student, admired by the male students, she lives alone because her parents work overseas. The city she lives in is ZeroPolis, an artificial maritime city operated by MegaLink, the world's largest international megacorporation.
But lately this city has been hit by a string of bizarre, thug-like and gruesome incidents.
Amid this life, one night Reina witnesses a girl fighting a strange monster in a corner of the city. Seeing the girl in trouble, Reina rushes to help without regard for her own safety. At that moment, Reina picks up a card that fell from the girl's bosom.
Suddenly she is enveloped in a blinding light and transforms into a beautiful, pure warrior. Reina is startled by her own appearance.
The girl is also surprised by Reina's transformation, but then calmly gives Reina instructions, and Reina keeps using the mysterious power, following the girl's directions to defeat the monster.
After the battle ends, the girl approaches the stunned Reina. The girl begins to explain what happened. She claims to be a homunculus created by alchemy, who fled another world after being ordered to stop a certain something from falling into the hands of an evil empire.
That something is exactly the mysterious card now in Reina's hand... it is a magical card called the "Angerium Card" that houses the power of angels.
But the evil empire seems to have already begun invading the surface, and the monster from before was one of its minions. The girl asks Reina to use the card's power to protect the surface world from the evil empire's invasion.
At first Reina refuses, feeling she cannot fulfill such a heavy mission, but soon, with confusion and a strong will to protect justice and peace on the surface, she accepts the girl's earnest request...