Super Devil is a special mode (and rank) where you can play as any character with green skin and powerful weapons.
In the game's respective Combat School, having the fastest runs in Pin Point and beating Survival Mode will net the player with the max score of 1,000 points. Reaching this amount raises the rank ladder from "General of the Army" to "Super Devil" (this rank is hidden when below one thousand points). At the Super Devil rank, Sophia or Margaret will start expressing an enormous amount of admiration to the player to the point of excessive love.
When in Survival Mode, the player character appears with green skin (sometimes purple skin). Their default pistol is changed to shoot powerful Slug Cannons (or Super Grenades in MSX). While this eases the difficulty, the player character still dies in one hit resulting in a complete restart. Collecting any special weapon replaces the Super Devil pistol, a problem since the special weapons have infinite ammo in Survival Mode. Players also keep their weapon in transitioning to the next stage. Becoming a mummy reverts the Super Devil pistol to a standard pistol.
Super Devil Mode is only available to the profiles that have the rank. There is no way to revert back to standard Survival Mode unless a new profile is made.
Collecting 23 Rank Medals in Very Hard difficulty awards the player with the Super Devil rank. The player is given invulnerability for the rest of the game, the exceptions are being crushed, falling on a pit, or landing on spikes. The only way to turn Super Devil Mode off is to start the game from the beginning as selecting S-Continue keeps the rank.
Rescuing all one hundred prisoners awards the player with the Super Devil rank. As a bonus, pressing down at the character select screen unlocks Hero Mode. In this mode, the player characters ditch their guns for powerful punching gloves that are able to knockout almost everything in one swing and gain the ability to throw infinite grenades. As a compromise, they also die in one hit, costing a retry.
Hyakutaro is ranked as a Super Devil in this game; he's ranked as a Second Lieutenant in every other game.
Super Devil versions of Marco, Tarma, Eri, and Fio appear as purchasable units when the player hits 10,000 points on the Co-Op Ranking Event. Each Super Devil costs 10,000 points, needs 999 AP to deploy, and has 2,500 HP at level 1 (going up to a maximum of 12,500 at level 40). Unlike other units, only one Super Devil can be placed in a deck at a time.
The Super Devil this time becomes an online rank in the game, being the highest ranking which requires a total of 3,000,000 points or above to get this rank. The Super Devil rank doesn't have any physical changes or attributes to any of the units in the game.
Super Devil units became available starting with the "Green Monster" event. Unlike in Metal Slug Defense, each unit has their own ways of attacking. The events' cutscenes also show the different manners a character becomes a Super Devil.
- Super Devil Eri is a pre-acquisition unit for the "Green Monster" event. She replaces her Slug Cannons with Hand Piercers that travel at a very long distance. In the event's opening cutscene, Eri transformed into a Super Devil after a green Hazmat Soldier rolled a bowling ball grenade at her.
- Super Devil Marco is a pre-acquisition unit for the "Maximum Evolution" event. He no longer charges forward when firing his Slug Cannons. Marco transformed into a Super Devil when a stray bat dropped a cauldron on him. He returned to normal after another bat dropped a vial on him.
- Super Devil Tarma is a pre-acquisition unit for the "Last Resort" event. Tarma was possessed by the Super Devil. He returned to normal after the Super Devil completed its task.
It's revealed that the Super Devil is apparently an entity of some sort, as Sophia states that it only comes when a crisis is about to hit the Regular Army.
- Sophia can be seen wearing a devil outfit in Metal Slug's concept art.
- Margaret refers to her students as "Greenhorns" (a novice), which the Super Devil has in its insignia.
- Touching Margaret's chest in Metal Slug Defense in any other rank causes her to berate the player. As a Super Devil, she sees this as a form of flirting.