Full Metal Panic! Sigma

A sergeant by the name of Sousuke Sagara is assigned an important task. That of guarding high school girl Chidori Kaname. Sousuke is a military genius, gaining high rank at an early age, but because of his strange upbringing, he lacks social skill. Now, he finds himself as a fish out of water as he attempts to protect a girl from an evil organization and turn in his school work on time. (Source: Anime News Network)

It's been twenty years since that fierce final battle...Sousuke and Kaname are husband and wife, as well as loving parents to their children, Nami and Yasuto. A life of peace and quiet...is the last thing you'd ever see from this family! When you have parents like this, every day is bullets and bombs galore! (Source: Yen Press)

Sagara Sousuke isn't your typical high school student. He reads military enthusiast magazines; he responds to questions with "affirmative;" he brings grenades to school in his bag. Though everyone at school takes him for a hopeless military geek, Chidori Kaname thinks there might be something more to him. When their plane is hijacked in the middle of a field trip, Kaname's instincts will prove correct: Sousuke is an elite, mech-piloting mercenary... and he's here to protect her! (Source: J-Novel Club)