Full Metal Panic! Volume 2

Two months after Sunan, Sousuke and Kaname are in a holding pattern. He'll fire his rifle off the school roof; she'll yell at him to stop; lather, rinse, repeat. That all changes one day, when Sousuke's commanding officer Teletha "Tessa" Testarossa shows up at his apartment, trying to hide a key to a terrorist superweapon of unprecedented power. Tessa is brilliant, pretty, and the same age as Sousuke, and her presence might put a serious crimp on his relationship with Kaname... assuming the city survives the night!

[From J-Novel Club]