Ayako and 40 classmates are suddenly blown to the other world by the power of the girl. There, they are have to fight against the zombies.... Ayako goes to various war-related departments, such as assault soldiers and sanitary soldiers, to go to war and to go back to the former world - - !!

[From StudioRobb]:

This is the story of a teenage boy named Takeru Tachibana and his struggle to recover his sister Sayoko from a "cyborg dogfight circuit". When Takeru was 10 years old, his town was bombed during an enemy air-raid in which his parents were killed and he was mortally wounded. Sayoko, cut deep with "survivor's guilt" and a "co-dependent" inability to deal with living alone, rashly sells her brain to the Albion Corporation run by a mysterious CEO named Véltare Mar?eau in order to pay for a risky surgical operation that Takeru may or may not survive. Her brain is placed in a cybernetic body, the process of which essentially eliminates the brain's memories and heightens its aggression. Her new identity is The White 13, or "The Ice Doll", and for the next five years she is placed on a dogfight circuit fighting other cyborgs in gladiator-style extravaganzas where she eventually becomes the grand champion and obtains the rank of "sirius"...

The story begins shortly after Takeru has liberated his sister from the dogfight circuit only to discover that, outside the cicuit, Sayoko's brain has a "self-destruct" mechanism and that she is dying. Takeru has one week to find a way to cure his sister or return her to the circuit or she will die. Adding to the problem are the "dogged" efforts of the Albion Corporation to recover "their stolen property" as well as a cast of characters who all have their own scars of some sort...

Koharu is in training to become a warrior goddess, but lately she's been restless. Being raised in a Goddess Palace means she's never seen men before, and like most 15-year-old girls, she has a keen interest in them. After finally escaping to the outside world, her superiors agree to let her stay there under supervision. But a showdown with Yusemi, the evil force that Koharu is destined to face, looms.
(Source: RebelliousArt)