Wild Arms Crossfire

Flower Thieves takes place thousands of years after a war between humans and demons destroyed much of the life on the planet, turning the world into a scorched wasteland. Set in a dystopian future, the manga features a large group of humans on their last leg, living in the overcrowded city of Upper Hose where flowers and other flora are rare and valuable. When a plant-eating monster known as a "Flower-Thief" attacks a mysterious girl named Jechika, a young boy, Maxi, must use a forbidden ARM weapon to save her, and is subsequently expelled from the city for using illegal technology. Traveling into the wilderness with Jechika and a florist named Gi, Maxi sets off on a quest to restore the balance of nature throughout the world and make the earth habitable again. (Source: Wild Arms Wiki)

Jude Maverick has grown up in an isolated town called Ciel, which is completely enclosed in a large sphere floating thousands of feet above the surface of Filgaia. His sheltered world changes forever when he sees the sky "tear" and ships enter his homeland. Upon inspecting the campsite set up by the intruders, he sees something else that he has never seen before: a girl. The girl's name is Yulie and she is being held captive by the intruders after being captured by a "Drifter" for hire, Arnaud G. Vasquez. After the invading army attacks Ciel and brings the sphere crashing down, Jude, Yulie and Arnaud join forces to find the army's true intention behind Yulie's abduction. With the help of another Drifter, Raquel, these four idealistic teens travel the war-torn land of Filgaia in search of truth, their own identities, and their separate paths to adulthood.