Full Metal Panic

Sagara Sosuke may look like a high school boy, but he is anything but your average teenager: hardened by years of battle, he is now one of the most skilled (and humorless) operatives of the secretive and powerful paramilitary MITHRIL organization. All this changes, however, when he, along with the lecherous sniper Kurtz Weber and ornery commander Melissa Mao, are assigned to protect pretty young Chidori Kaname, who may be one of the mysterious and sought-after "Whispered". Trained assassins, mech-piloting psychopaths, and booby traps, Sagara can deal with... it's infiltrating a Japanese high school and trying to blend in that's the real challenge. And he will learn that an enraged girl can be the most lethal opponent of all.