Captain Levi, whose real name is Levi Ackerman, is the captain of the Special Operations Squad within the Survey Corps and he is considered humanity’s strongest soldier against Titans; despite this, he’s quite a short and thin man, so we can think of him as the embodiment of the phrase “big things come in small packages”.
In addition, as far as his personality is concerned, Levi is easily irritable, short-tempered and difficult to approach; he never smiles and he never cries because he doesn’t want his emotions to distract him from his tasks. He’s a very realistic and pragmatic man who expresses no fear and no anxiety, since he knows his stuff and he’s confident about himself and his abilities.
In the spin-off chapter titled “Attack On Titan: Birth Of Levi” we can comprehend that he comes from the Underground District, where people live in completely disastrous conditions, being abandoned to themselves; in fact, before becoming the person we see now, Levi was a simple thoug and probably this previous part of his life allows him not to have consideration for any social tact, meaning that if there is something to be said, he never shields anyone from “hard facts”. Moreover, before becoming an high-ranked soldier, Levi lived a shocking moment in which he lost his friends; in my opinion this is the fact that made him different from any other person: he is different because he has already been broken, and someone who has already been broken, knows how to broke other people.
Furthermore, he feels disgusted by unnecessary deaths, especially because his fellow soldiers are the only things he openly displays care for; in fact, he deeply cares for his allies and this is probably the main reason why he has no mercy for Titans: a mixture between a revenge feeling for his past and a protective feeling for the present.
In conclusion I believe that it’s important to underline that, despite he’s the strongest human fighter against Titans, he is still capable of making mistakes; this fact can remind us that, although you give your very best, that may still not be enough, even for a proper hero like Captain Levi Ackerman.