Key Items are special items that can be gotten by achieving certain milestones in the game, such as finishing all missions or Fulton extracting a certain number of things. These are usually required in order to develop the more elusive items, and they are also needed in order to get the Collector achievement/trophy . The following list will show what you need to do to get the Key Item, as well as what it unlocks. Remember that the item that unlocks has to be developed in order to become available.
Complete Mission 18.
First Aid Manual
Found in Masa Village after Mission 18 (see image below).
Tactical (Fulton) for D-Dog.
Master Certificate (Marking)
Mark 500 objects or people with your binoculars.
Enemy decoys are now marked with a white square.
Master Certificate (Fulton)
Extract 500 enemies.
3-Star Fulton Device.
Grand Master Certificate (Fulton)
Extract 1000 enemies.
Master Certificate (Box)
Collect all invoices in either Afghanistan or Africa.
Grand Master Certificate (Box)
Collect all invoices in both Afghanistan and Africa.
Complete Side Ops 113.
Children upgrade for Fulton Device.
Complete Mission 29.
Task-Arm SM (D-Walker).
Complete Side Ops 143.
Man on Fire (Corpse)
Complete Side Ops 144.
Furicorn Uniform (D-Horse).
Quiet's Examination Report
Complete Mission 40 or view the cutscene where Quiet gets interrogated at Mother Base.
Gray XOF Uniform (Quiet).
Complete Mission 40.
Sniper Wolf Uniform (Quiet).
Emmerich's Research Notes
View cutscene where Emmerich gets exiled from Mother Base.
Master Certificate (Standard)
Complete all normal missions (any rank).
Cyborg Ninja Uniform.
Grand Master Certificate (Standard)
Complete all normal missions with an S Rank.
Master Certificate (Elite)
Complete all high-difficulty missions (any rank).
Naked (Silver) Uniform and Bionic Arm (Silver).
Grand Master Certificate (Elite)
Complete all high-difficulty missions with an S Rank.