Full Metal Panic!

Sousuke Sagara is a high school student with unusual habits, such as carrying grenades and reading military publications. His behavior leads others to see him as a military enthusiast, but Chidori Kaname suspects there is more to him. During a school trip, when their plane is hijacked, Kaname's intuition proves right—Sousuke is a skilled mercenary who pilots a mech and is assigned to protect her.