Full Metal Panic!

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)

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)

Tells the same story as Full Metal Panic Second Raid and is as such, also based on the novels The End Of Day By Day. Leaving off after the original series, this series tells of Sousuke and Mithril as they fight Amalgam. Like the original series, but both sides have gotten a bit more aggressive...

Tetsuro Kasahara manga adaptation of Shouji Gatou eponymous novel. *Note: This adaptation interrupted publication after a disagreement between the original novel author and the artist of the manga version over Gatou's disparaging comments of Swedish environmental activist Greta Thunberg, leading Kasahara to claim he will no longer promote Full Metal Panic! Zero nor draw any continuation, and will instead donate any profits from the manga to environmental groups.*