A Rank Medal is a special collectible in Metal Slug: 1st Mission.
A Rank Medal appears as a orange shield with sparks. When a mission is cleared with a Rank Medal in hand, the player's rank and total health is increased. Getting a Game Over takes away the Rank Medal from the player. While a mission can have multiple Rank Medals, only one can be collected at a time, and the other medals disappear until the mission is cleared (or the player dies).
In Very Hard, collecting Rank Medals is the only way to increase ranks; lower difficulties can also increase ranks if the player does well in a mission. As a compromise, collecting the 23rd Rank Medal through various playthroughs (by using S-Continue) grants the player with the Super Devil (SD.) rank and permanent invincibility. The only things that can hurt the player at this point is being crushed, falling down pits, and landing on spikes. Super Devil Mode stays on until a new game is played.
In other difficulties, the player can only reach the 22nd rank: General of the Army (GA.). The player's maximum health is the same regardless of difficulty.