ts eiseihei-san no senjou nikki

TS Eiseihei-san no Senjou Nikki (lit. TS Medic's Battlefield Diary) is a Japanese web novel turned light novel series written by Masaki Tama and illustrated by Kureta, with a manga adaptation by Mimishiki. The series is an isekai set in a WWI-ish war with magic. The TS in the title is short for Transsexual, which refers to the Transsexual Fantasy genre, ie series involving genderswapping.

The story follows Touri Noel, a 15-year-old girl who was a male in her past life and a skilled FPS player. After learning she could use healing magic, she left the Noel Orphanage and volunteered to join the military following a childhood friend. Assuming she could make use of her FPS knowledge to succeed on the battlefield, she quickly learns the reality of war is a very different and very brutal thing as she tries to stay alive.