Full Metal Panic! Sigma

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)

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)

This is another story in the Full Metal Panic universe, set some years after the events with Sousuke and Kaname. Ichinose Tatsuya is a pretty normal high school boy, who lives with his father and little sister Yukari. He has a tendency to fall in love with foreign women, which has never turned out well for him. One day, while up in the mountains driving a Power Slave to help out with his father's company, he runs into yet another beautiful foreign woman, called Adelina. He lends her a hand and is surprised to see that she has a military Arm Slave, called "Shadow," with her. What is this mysterious and highly armed woman doing up in the mountains? Tatsuya's life is about to get a lot more exciting... \[Written by AkatsukiUlquiora@MAL\]

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)

Basically, it's a retanking of some chapters from Full Metal Panic!. It's sort of like FMP! Fumoffu.. meaning that there's no mecha plotline, only short stories. Also there's an extra story not presented anywhere in each volume of this series.