Armin Arlert (アルミン・アルレルト) is a soldier of the Scouting Legion, high-ranking member of the Shota Gang, and the deuteragonist of the series, which is a word you probably can't even pronounce because you aren't as smart as Armin. It means second most important person.
Once upon a time, he was also the son of Annie_SnK's writer.
He has been given a wide range of nicknames, much to Armin's chagrin. Recently, he has formed a strong aversion to the nickname coconuthead, taking offense to being associated with small, round foods. Rumors say he won't even eat such foods anymore.
Armin is wrongly described as a crybaby. He is a badass strategist, but cries when he feels weak because he doesn't know what he can do. Talk about underestimating yourself and having no confidence. He also uses emotional blackmail on Annie, on episode 23, when she pretty much laughed her ass off, and I'm sure I heard people actually fapping.