Attack on Titan: Lost Girls (Novel)

Attack on Titan: Lost Girls (進撃の巨人　ＬＯＳＴ　ＧＩＲＬＳ) is a Japanese novel and compilation of three stories written by Hiroshi Seko,  who worked as a scriptwriter for the anime. It's based on the visual novel of the same name with the addition of a new one, Lost Girls, which details Annie and Mikasa's relationship in the training days. Mikasa wonders why Annie acts the way she does and what burden she carries.