Usually if you play easy difficulty in Metal Slug 6 you play up until you get to a cavern.But if you play on hard mode you will get to see the true ending. (And final battle)Also you get to do another level.
In mission 4, 5 aliens will be in a formation moving left, right, and down, it is a reference from the classic arcade game Space Invaders.
Unlockable How to Unlock
Special Music-Complete Metal Slug 1-5 and X on Hard with limited continues and also completing Metal Slug 6 with limited continues
Tonko Art Gallery-Complete Metal Slug 1-5 and X on Easy with limited continues and also completing Metal Slug 6 with limited continues