Armin Arlert is Eren's best friend. He is physically weak, but emotionally strong. Even though he lacks in actual combat skills, Armin is a genius in theoretical courses and can make plans even under extreme pressure. Even so, Armin keeps 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 on Eren and Mikasa.
(Source: Wikipedia, edited)
Note: Armin Arlert is the official English translation by Kodansha.
