All basic information based on Levi's Wikia page.

Back story based on A Choice With No Regrets/Birth of Levi, a Levi-centric spin-off written by Gan Sunaaku and illustrated by Hikaru Suru. It's based on the visual novel of the same name.

Levi grew up in the Imperial Capital's Underground City, a district located beneath Wall Sina. It was initially developed in the event of Titan invasion, but the project was abandoned. The remaining structures are now occupied by criminals and the poor. Reconstruction by the inhabitants made the Underground City dangerous for the Military Police to enter. Needless to say, it's a lawless and desolate place, a contrast to the prosperity aboveground.

• Levi has no family. Instead, he has allies. The closest he had to friends before the Survey Corps were Isabel and Farlan.
• The 3DMG they used was acquired from the black market or stolen from soldiers who dared to enter the Underground. The ones from the black market were beyond their service period.
• Levi began mastering 3DMG and martial arts as a teenager. Along with malnutrition, his body remained small, but flexible and compact with well-developed muscles. He takes no offense in being called short, and credits his small stature for his efficiency in killing Titans.
• Levi doesn't know his own age. Upon enlisting, he merely gave a rough estimate to the records office. On paper, he's 28 years-old.
• He doesn't know when his birthday iseither. Erwin and Hange mutually agreed to celebrate Levi's birthday on December 25 to coincide with The Savior's birth.
• His eyes usually appear slate gray because of the shadows cast by his expression and the viewpoint of taller people. His eyes are actually icy blue with flecks of navy, which can only be seen from a lower angle.
• Before being recruited in the Survey Corps, members of the Military Police have attempted to capture Levi several times. During the first attempts, the objective was to apprehend him and let him stand trial. However, Levi's fighting skills and the Police's unfamiliarity with the Underground caused many soldiers to die. Eventually, the objective (unofficially) changed to killing Levi on the spot. The most fatal encounter came when a rookie soldier shot him at mid-range with a musket.
• Though the bullet didn't kill Levi, it was lodged in his intestine and caused a heavy amount of internal bleeding. Because there were no doctors Underground, his only choice was a former taxidermist. Along with the lack of proper equipment, the instruments used were filthy. Between the barbaric attempt at surgery and waking a few days later, he doesn't remember much because he was delusional with a fever that was closer to killing him more than the bullet.
• Levi has a scar in his right side. The entrance wound is around the size of a coin and is located in the central area of the abdominal fourth quadrant. The exit wound is located at the back and measures nearly a hand's length. The scar tissue plugging the exit wound is shiny and deformed, not only because of the graceless surgical procedure but also because of the subsequent infection. Though the scar doesn't affect Levi's mobility, it aches during very cold weather, making him even more irritable than usual.
• He has an obsession with cleaning because of the incident that nearly took his life. Aside from that, he has also survived a few plagues in the Underground wherein the most effective defense was boiling food and washing one's hands frequently. For Levi, clean means safe.