Eren's best friend, he is physically weak yet emotionally strong. Even though lacking in actual combat, Armin is a genius on theoretical courses and can make plans even under extreme pressure. Even so, Armin kept on thinking that he is weak. He himself does not notice the things he does for Eren and Mikasa, and believes himself to be completely incompetent and hates himself for having to rely of Eren and Mikasa.
Note: Armin Arlert is the official English translation by Kodansha.