It's no secret that Levi Ackerman led a terrible childhood in the slums underneath the capital. He had to fend for himself ever since he was still a small boy and more often than not, he had to turn to crime just to survive and live for another day. In his youth, violence was not only a means to an end, it is basically a day-to-day lifestyle for Levi. He wouldn't hesitate to beat up or even kill someone if he thought it was necessary to do so.
So it's no wonder he ended up growing to be such a stern and ruthless man. There are only a handful of people that he considers to be close friends, and he is willing to do everything he can to protect them. For everybody else, he's been shown to be less sympathetic. So how come such a callous man ended up becoming Humanity's strongest soldier? Why did Levi decide to join the Survey Corps in the first place? A spin-off manga called Attack On Titan: No Regrets was created to answer this specific question.
Highly Proficient In 3D Maneuvering Gear
While still living within the cave of the underground slums, Levi and his two friends somehow manage to get their hands on 3D Maneuvering Gear. Despite nobody teaching them how to use it properly, they manage to be quite adept at it. Levi in particular quickly becomes capable of using the gear as if it is just an extension of his body.
Thanks to that, their life becomes much easier. They can go anywhere within the cave much faster than ever before, and if there's anybody that dares to challenge them, they have a means of protecting themselves. Without realizing it, Levi and his friends have become much more proficient at using these gears than average soldiers within the Survey Corps itself.
Recruited By Erwin Smith
It goes without saying Levi and his friends quickly became the talk of the town. Those whispers somehow end up making their way into Erwin Smith's ears. Erwin is not the commander of the Survey Corps at the time, but the higher-ups recognize his leadership ability, and it is only a matter of time before he climbs to the top.
When Erwin heard about a bunch of thugs using 3D Maneuvering Gear in the underground slums, he became highly interested. So rather than catch Levi and his friends on the spot, Erwin decided to see what they could do first. He took several experienced soldiers from the Survey Corps and chased Levi's gang throughout every nook and cranny within the underground city. That is when he realized that Levi's talent was indeed the real deal.
Erwin may not be able to catch Levi on his own, but his team manages to apprehend Levi's friends quite easily. Erwin knows that in order to venture outside the walls, he needs every talented person that he can get. So rather than putting Levi and his gang in jail, he recruits them into the ranks of the Survey Corps instead. Needless to say, Levi is reluctant at first, but since Erwin used his friends as bargaining chips, Levi doesn’t have a choice but to accept Erwin's proposal. And just like that, he becomes a member of the Survey Corps.
Experiencing Titans For The First Time
During their first expedition outside the walls, Levi, his friends, and all the other soldiers of the survey corps were actually on a trial run for Erwin's new formation plan. This formation ensures the highest chance of survival for every soldier due to its enhanced communication technique. Since Levi and his gang were criminals from the slums, other soldiers often made fun of them and underestimated their abilities. That is of course until the group encounters their first Titan.
As luck would have it, their first enemy is an abnormal Titan. Several of the team members die trying to subjugate the Titan. The sight of their comrades being devoured alive sends shock through the remaining soldiers on the team. However, Levi is the only one unfazed and simply observes everything. Once he notices a behavioral pattern of the Titan, he and his friends move in for the kill. His two friends attack the leg of the Titan while Levi goes for the nape. In an instant, Levi gets his first Titan kill along with the fear and respect from other members of the Survey corps.
A Change Of Heart
By this point, Levi is still a reluctant member of the Survey Corps. He merely goes along with Erwin's plan in order to ensure the safety of his friends. But truth be told, Levi actually concocted a plan to kill Erwin during this expedition. However, before he gets to do that, the whole landscape is covered by a thick blanket of fog. Levi decides to leave his friends and the rest of the team to find Erwin.
Unfortunately, not long after he leaves, several Titans come and attack the team. He rushed to help them but by the time he arrives, the Titans have killed everybody, including Levi's friends. Filled with rage, he slaughters every Titan in the vicinity until Erwin comes and tries to calm him down. At the moment, Levi is ready to kill Erwin because he blames him for forcing them to join the Survey Corps.
Levi blames Erwin for the death of his comrades, and himself for his inability to protect them. But Erwin says otherwise. It is not his fault or Levi's fault, it is the Titans' fault. That's why they need to be eradicated, so that everybody can freely walk outside the walls without being afraid of being eaten alive. Levi's skills play a key role in Erwin's vision. This is the moment when Levi finally accepts and understands Erwin. It's also the time when Humanity's strongest soldiers are officially born.
