Attack on Titan: Lost Girls (Novel)

Attack on Titan: Lost Girls (進撃の巨人　ＬＯＳＴ　ＧＩＲＬＳ) is a Japanese novel and compilation ot 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.

Publication
Kodansha published it in December 2014 under their Kodansha Comics Deluxe imprint.

North American publisher Vertical announced their license to the novel at Anime Expo 2015.