 If you liked
Full Metal Panic! Sigma
|
 ...then you might like
Muv-Luv Unlimited
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These two manga here both involves dealing with the military and the use of mecha in combat.
Both series features a prominent main characters who gets involved with the military. From there, he befriends other people including a main female protagonist whom later falls in love with them.
Both series has violence, some fan-service, action, drama, and comedy.