‘Gun x Sword’
Armed with a metal sash that straightens into a sword, lanky swordsman Van is traveling through the desert, searching for the mysterious clawed man who killed his bride. Along the way he picks up a fellow traveler: Wendy, a young girl whose town he saves from bandits by summoning a mecha with his sword. Wendy is searching for her missing older brother, whose engraved gun she carries on her back. In their mutual quest they run afoul of more bandits and villains, a mysterious woman named Carmen99, and more mecha battles a la Wild West.
Descriptions taken from AniDB.net