Sousuke Sagara, a 16 year old mercenary, who has lived all his life fighting in various wars. His next mission? To protect a high school girl. This anime combines elements from different genres to create something very enjoyable. There's action, mecha, comedy and some romance, so you'll definitely find something you like.
One of the main characteristics of the show is how it changes from the funny sequences in high school, where Sousuke tries to fit in but he can't due to his soldier mentality, and the serious action scenes where Sousuke pilots his robot and stops the enemies.
And the other main aspect of the show is the relationship between Sousuke and Kaname Chidori, the girl he must protect. Sousuke always takes things literally and can't seem to understand everyday logic. Chidori is short-tempered, so she always ends up yelling at Sousuke for his abnormality. But as they grow closer they start to slowly understand each other.
The characters are really likeable. Besides from the two main ones, you find Sousuke's co-workers and a clumsy girl around his age who happens to be his captain.
As the story progresses it becomes more serious. I recommend you to watch it. Also, Sousuke's awesome!