Armin Arlert

Age: 15,
19 (after time skip)
Birthday: November 3
Height: 163 cm (5'4") -> 169 cm (5'6")
Weight: 55 kg (121 lbs) -> 56 kg (123 lbs)
Affiliations: 104th Trainees Squad, Scouting Legion
Graduation Rank: Outside the top 10

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.

After the battle of Shiganshina District, he took the power of the Titans from Bertolt Hoover and came in possession of the Colossus Titan.
(Source: Wikipedia)
Note: Armin Arlert is the official English translation by Kodansha.