Kourin Tenshi En Ciel Rena

Rina Kanoe is a diligent girl student who lives alone at the Zero Police artificial city controlled by the largest international corporation. However, there have been a lot of demon appearance incidents lately. One night she witnesses a girl fighting a monster and getting defeated. Rina picks up a card that fighter girl drops and transforms into a pretty warrior girl. Rina manages to beat the monster by following the defeated girl instructions. The girl turns out to be a runaway from another world. No one else is able to use these magic cards, so Rina agrees to stand up and protect justice in the city.

[From vndbreview]