Time: Levi joins the Survey Corps in 844 to the establishment of the 104th Training Corps in 847.
Kirna witnessed Levi violently blow open a man's head with a gunshot. At the same time, she also had no home. Why did she join the Survey Corps? Probably to meet Levi and tell him——
Danger: privately designed lethality, 99% original plot
Purpose: To open a tower for Levi's happiness