Armin Arlert is solider serving in the scouting legion, in the anime Attack on Titan. Armin is short and generally weak, he makes up for this with with intellect. His snap decisions and quick plans, have secured many victories against the titans.
Reason For Cross-Dressing
Officially there is not much content for Armin being a trap. He thought to be a girl by a interview with Attack on Titan creator Hajime Isayama himself. Where he said to the question, “Out of the girls in the 104th, who is Mikasa closest friends with?” and he responded, "That’d be Armin". In another 4koma they joke that Armin would look the best in a rabbit costume, and while only chibi he did crossdress which lead to many artists doing fan art of this scene. It's commonly referred to as "Usamin". He also crossdressed in the spin-off series "Attack! Titan Junior High". Later on in the true series he also dressed as a female co-star named "Krista".