Levi Ackerman, Captain of the Survey Corps, is often referred to as the strongest soldier in Attack on Titan and has one of the most avoidant personalities when it comes to socializing. But, he is also one of the series' most protective and respected characters. Despite Levi's antisocial and unapproachable tendencies, his kind heart derives from his passion to protect humanity.
Levi takes the lessons he learned from growing up in the Underground City and from working his way up in the Corps and uses them to guide others. He values human life over everything, and, even as he learns of the shortcomings of the world, he keeps his morals and ethics sharp so that he can be nice to those he cares for and keep humanity intact.
Titan Serum
Armin Alert didn't start as the most brawny soldier when he and his friends joined the Survey Corps. Armin was always respectful to his superiors, and Levi's fighting skills quickly earned him Armin's trust and respect, leading to an unexpected friendship. Due to Armin's ability to formulate good strategies and his deductive reasoning, Levi also developed a certain level of respect and understanding for the young soldier.
Levi was honest and up-front with Armin after he felt guilt for his first kill to protect Jean. Levi assured Armin that it was beneficial to make sure everyone in their regiment survived. Levi's knowledge of Armin's ability to handle the power of being a titan and knowing that Erwin could finally be at peace played a factor in his decision to later give Armin the titan serum.
Stopped Eren's Execution
When a somewhat recognizable titan appeared and started helping the humans during the Battle of Trost, Eren's friends and fellow soldiers realized it was him after he returned to his human form. Armin and Mikasa obviously wanted to protect Eren, but Captain Kitz Woermann and Commander Nile Dawk were both in favor of his immediate execution.
Levi reassures the military leaders and politicians that he can keep Eren "in check" and that it will benefit the Scouts and humanity, in the long run, to allow Eren to stay active. Even by making an example of Eren and physically hurting him in the courtroom, it was just an act and in AoT, protecting a comrade and all of humanity is one of the nicest things anyone can do.
Eren's Trust
When the Survey Corps was facing the Female Titan in the woods, Eren was bombarded with the encouragement of his teammates to remember that Levi knew what he was doing and that they should all trust in their captain. Eren knew this, of course, but also had only recently discovered his Titan powers and wanted to protect everyone he cared about.
It was when Levi finally stepped in and reminded Eren that he could either put his trust in only himself or in his fellow squad members. Eren chose the latter and continued leading the Female Titan away so that they wouldn't lose any more soldiers.
Fallen Comrades
One of the saddest things about Levi in Attack on Titan is that he cares so deeply about protecting humanity that even his own decisions have led him to lose important people his entire life. He kept his fallen comrades' badges in his desk drawer, and, even though it isn't a gesture anyone but him can appreciate, it is a nice gesture nonetheless.
In the episode "The Defeated - 57th Expedition Beyond the Walls," Levi's entire squad was killed by the Female Titan. He approached the piles of bodies and took a badge that belonged to Petra Rall, the only woman on his regiment. While the other leaders wanted to keep the deaths labeled as those soldiers missing in action, Levi presented the badge he retrieved as Ivan's to one of his soldiers so that he could share that keeping the badges reminds him that they were alive and human.
Zeke's Wine
In season 4, episode 73, Levi faces Zeke Yeager and all of his comrades that drank Zeke's wine that was infused with his spinal fluid. Levi knows that he swore to protect humanity, and, before this battle, he learned that humans have been turned into Titans long before this.
Zeke even revealed his plan to Levi, and Levi decided then that Zeke had no regard for human life. His deductive reasoning, passion to protect what was left of humanity, and knowledge that his comrades didn't have a choice in this new life led him to kill them all.
To The Sea
After Armin received the Titan serum, Levi had overheard Eren and Armin talking about seeing the ocean without one another, but, when Armin survived, so did their dream. This means a lot not only to Eren and the others, but to Levi's own humanity, as he reminisced on Kenny's words about people clinging to dreams. Levi knows that these dreams are what his soldiers, and friends, lean on in dire times. He helped them achieve that.
Friendship with Erwin
Levi doesn't typically allow himself to get too interpersonal with anyone, and, for a while, Attack on Titan's oft-mistaken Erwin was no exception. Levi would still use his unbothered and somewhat snarky tone of voice in conversations with his commander. Yet, after some time together and Erwin gaining Levi's trust, they were able to banter back and forth, work well together, learn from each other, and even consider one another friends.
Takes on the Beast Titan
During the return to Shiganshina, Levi tells Erwin to just save as many lives as possible and use Eren to their advantage. That distraction would allow Levi to take on the Beast Titan on his own. This is a moment of bravery, but it's also a moment of protecting his friends.
Levi would rather the Survey Corps survive with Eren and Erwin rather than lose them all. This is one of the nicest things Levi did because he went into his one-on-one battle knowing he could very well die. He also did it to benefit Eren, someone he originally didn't trust and didn't entirely believe in.
Handpicked His Squad
For Levi to handpick soldiers was an unexpected and nice gesture, but it was one that would prove to benefit the fight against Titans and corruption. He developed a relationship with everyone in his squad before he was able to bring them together to create his own special operations group, also known as the 'Levi Squad.' He also built this team to keep an eye on and protect Eren Yeager and Historia since they were valuable to the team and the understanding of the political involvement of the creation of Titans.
Bringing together Eren, Mikasa, Armin, Sasha, Connie, Petra, Jean, and a few others benefited the team, their bonds, humanity, and Levi's ability to trust his fellow soldiers. In the end, the elite team he chose had over 200 Titan kills and was known to be able to take out nearly any type of Titan to cross their paths.
Protecting Humanity
Above all, Levi Ackerman has been protecting humanity since he became a powerhouse through the titan experimentation his family experienced. With enhanced physical and mental abilities for swift judgment and unmatched combat skills, Levi does whatever it takes to win against Titans and those that would stand in his way of protecting Paradis.
Levi uses his power—artificial and otherwise to fight his hardest—sacrifice himself for the greater good, and lead those that he believes will be the protectors when he is gone. His morality is the nicest thing about him.