Attack on Titan's side characters are worthy of being shonen protagonists in their own right, with the hard-hitting Captain Levi Ackerman being an excellent example. Levi was born with nothing in the Underground, until his uncle Kenny found him and raised him to be a survivor and a fighter. Now, Levi is rising in the ranks quickly.
Levi seems like a bit of a loner, but not even he can always save the day all alone. Levi is smart enough to maintain ties with influential, powerful, and smart individuals who can back him up when necessary, with the science-minded Hange Zoe and Commander Erwin Smith being two fine examples. If Levi could foster relationships with other like-minded anime characters, who would be welcomed into his inner circle?
10 Shota Aizawa Is A Street-Smart Pro Hero (My Hero Academia)
When Levi is looking for friends, he's not exactly looking for a BFF; rather, he's interested in finding like-minded associates who can aid him, and who can relate to him on a personal level to some degree. Even if these characters don't join the Paradis Island military, they are still desirable, and Shota Aizawa/Eraserhead is a good start.
Eraserhead is a bit like Levi himself, being a tough but fair leader who will take any risk to confront evil and protect his allies and get the job done. Aizawa is street-smart and practical, ensuring that he could keep up with Levi and provide intel along the way.
9 Major-General Olivier Armstrong Knows How To Survive & Command (Fullmetal Alchemist: Brotherhood)
Major-General Olivier Armstrong is higher-ranked than Levi is, but Levi wouldn't associate with her just to kiss up to the top brass. He would be genuinely impressed by Olivier's "survival of the fittest" worldview and ability to command, and see her as a role model.
Olivier has a hard exterior but does care about her subordinates and allies, and Levi can identify with that. Olivier would trust and respect Levi in turn, and they would have a slightly aloof but respectful friendship all the same.
8 Naofumi Iwatani Is A Survivor Against All Odds (The Rising Of The Shield Hero)
The shield hero himself, Naofumi Iwatani, is a survivor who has endured several of the apocalyptic Waves already, and he didn't even have the other three Cardinal Heroes to help him — in fact, they were hostile to him. Levi would know a few things about facing endless waves of monsters that want to destroy everything.
Naofumi's personality is roughly similar to Levi's, and both shonen heroes would get along famously if they ever went on a mission together. They would definitely want to form more permanent ties just in case one of them ever needed the other's assistance again.
7 Neji Hyuga Is Ready To Fight Back Against Fate (Naruto)
Neji Hyuga of the Konoha 11 has some thematic ties to Levi Ackerman. Both characters were born into difficult circumstances, and they saw no way to reach a better future on their own. Neji, rather than being born in the Underground, was born into the subservient branch family of the Hyuga clan.
Neji learned to resist his fate, though, and fight for a better future. Levi is doing that too, to distance himself from his past and help the walled city be free of the shadow of the Titans. Neji and Levi would get along fairly well on a personal level too, considering they share a similar temperament.
6 Genos Takes Matters Seriously, Just Like Levi (One-Punch Man)
The "demon cyborg" Genos is more machine than man, but he's not at all evil. Instead, he has a heart of gold, perhaps literally, and he fights hard to protect the common people from monsters. That, and his extreme firepower, would make Genos a useful ally and associate for Levi.
As a bonus, Genos has a mature and practical personality that would greatly appeal to Levi, while Genos would greatly respect and admire Levi's ability to command and maneuver in battle in a dervish of blades. They're sure to get along.
5 Tetsuzaemon Is A Tough Guy With A Good Heart (Bleach)
Soul Reapers are the sword-wielding guardians of the afterlife, and their personalities vary greatly, from the fun-loving Rangiku Matsumoto to the stern and aloof Captain Byakuya Kuchiki. Somewhere in the middle is the fellow named Tetsuzaemon Iba.
Tetsuzaemon looks and acts like a stereotypical Japanese mobster, but he's a professional who fights hard for justice, and he likes to drink with his friends and spar with them. Levi can respect that, and the two of them will soon be like old war buddies. Levi probably won't want any yakuza shades of his own, though.
4 Mako Is A Street-Wise Firebender & Hero (The Legend Of Korra)
Mako and Levi aren't easy to approach, and if they met, they might just stay out of each other's way at first. But if a common enemy attacked, Mako and Levi would team up to defeat their foes, and they would form a positive impression of one another.
Mako is street-wise, mature, and practical, just like Levi, and while Mako is kind, he also isn't shy to scold troublemakers or talk someone down. Levi identifies greatly with that, and he would peg Mako as someone he can trust and respect, at the very least. They're almost two of a kind.
3 Hakuro Is A Wise Old Combat Veteran (That Time I Got Reincarnated As A Slime)
Hakuro may be much older than Levi, but that wouldn't stop them from becoming friends. Hakuro suffered the destruction of his entire home village at one point, with only him and a few other villagers surviving. Now, he will face any threat head-on to protect what little he has left.
Levi knows what that feels like, and he and Hakuro could swap war stories in between battles or sparring with the blade. Levi would admire Hakuro's wisdom and experience, and resolve to keep Hakuro in his inner circle of associates.
2 Captain Akitaru Obi Knows What It's Like To Lead (Fire Force)
Although Captain Akitaru Obi is a bit goofy and whimsical by Attack on Titan standards, he is still one of the more mature and sensible characters in Fire Force, and Levi would peg him as a true commander who would back him up in any fight.
Akitaru Obi can't use pyrokinetic powers, but he makes up for it with his tactical thinking and his experience with mundane equipment in battle. In turn, Akitaru would respect these qualities in Levi, and they could form a solid friendship based on mutual combat experience. They're both hardened soldiers.
1 Erza Scarlet Has Gone Through Similar Experiences As Levi (Fairy Tail)
Erza Scarlet is a powerful S-ranked wizard, but she was hardly born into greatness. Instead, she was kidnapped from her hometown as a young girl and dragged off to the Tower of Heaven project. There, she was forced to work as a slave among many other children. It seemed like Erza's life was already over.
But not so. Erza rallied her fellows and, with the aid of Jellal Fernandes, escaped. Now Erza fights to save others from similar suffering, and she has a mature and tough persona, like a big sister. But she has her kind side too, and Levi wouldn't mind adding her to his circle of trusted allies. They would understand each other on many levels.