The Crimson Harmony (Shingeki no Kyojin/Attack on Titan fanfic)

It was thought to be impossible to befriend a titan until the Survey Corps found a mysterious girl living in a forest outside the walls. The titan she had befriended was killed and after being named Ria, she was taken back into the walls. Ria was finally accepted and trained by Levi. She seemed to have a confused mind...