The Blacksmith refuses to give the Cultist the Bullet That Can Kill The Past, but it is also unnecessary to access The Cultist's past. Shooting The Cultist with The Gun That Can Kill The Past will access their past, whether or not the player has The Bullet.
- A controller has to be connected to be able to play co-op, otherwise the Cultist does not appear in the Breach. You can talk to the Cultist near the selectable Gungeoneers to start co-op mode.
- Enemy health is increased by 40%.
- There is an increased chance that a pickup spawns upon clearing a room.
- Guns can be traded between the protagonist and the Cultist via gun dropping.
- This is usually done by holding F for a keyboard or holding the "Down" direction on the D-pad of the controller.
- The same can be done with active items by holding G or "Up" respectively.
- Passive item benefits are only given to the player who picks up the item.
- Synergies can be shared between players.
- Synergy effects will work with both players. For example, Witch Pistol's Chicken Arise synergy will make all players have larger bullets when held.
- Dead players cannot do any standard player actions except moving and performing a short-range blank by pressing the respective button (or any button on console). The player turns gray once it's used and cannot use it again until they return to their normal blue color after several seconds. The dead player also can't use normal blanks, if that player has them stored.
- Players drop all of their guns and items on the ground upon death. These items can be picked up by the surviving player, and items left on the ground may be taken by the Resourceful Rat. Items like Master Rounds and Yellow Chamber will still award a heart container on pickup, but it will not be filled with a heart. This works for both players.
- There are three ways to revive a player in this mode. In every case the dead player will be revived to full HP.
- All unopened chests in the current chamber will turn into revival chests. If a revival chest is opened, the dead player will pop out of it, the chest will be consumed, and other chests in the chamber will revert to normal. Keys are not required to open these chests. It is recommended to check the chest's quality by looking at the map so that the players don't lose better grade chests for reviving, but instead using lower grade ones to do so. If a chest is destroyed, it revives the dead player without giving any pickups or Junk.
- Beating any floor boss.
- Using the Friendship Cookie.
Before the Cultist enters the Gungeon, their reason for being there is probably to make their sibling take the blame for spilling grape juice on the couch, as stated in the Ammonomicon entry for the Dart Gun. However, their motives change over the course of what happens in the Gungeon as they are treated poorly by the NPCs. Eventually, they come to the conclusion that what they want is to defeat player one and become the protagonist.
If The Gun That Can Kill The Past is used on The Cultist, they will be brought back to The Breach earlier that same day. The Cultist will tell the protagonist that they are sick of being referred to as "Number 2" and will then challenge them to a duel over who the game's story will focus on. Two pedestals each carrying a Magnum will have spawned, one behind the protagonist and one behind The Cultist. The two players are then allowed to move around The Breach, pick up a Magnum and fight each other to the death. Each player has 3 hearts and 2 blanks. The Breach is completely devoid of any of the usual NPCs at this time and all doorways, including the entrance to the Gungeon, are blocked off. A few tables are also scattered around, as to provide cover during the fight. If The Cultist wins the duel, they declare themself the hero only to realize that their actions could very well make them the villain.
Regardless of who wins the duel, the Cultist's alternate costume will be unlocked.
- If activated, the "increase speed when out of combat" effect will work for both players, since they technically are not fighting enemies and are at the Breach.
- The Cultist's blanks and armor will carry over into their past, giving them advantage over the protagonist during the final duel.
- Dead players can block moving bricks using their body in Winchester's minigame, stopping them.
- However, the chest that spawns will be a Revival Chest, and if the player revives their friend and returns, it will be locked.
- During The Cultist's Past, one of the players can take both guns off the pedestals and render the other player weaponless. The other gun picked cannot be used or dropped.
- If either player has stolen something, in the Cultist's past, the Cultist may refer to the main character as "thief", a reference to the Nintendo game "Link's Awakening".
- When entering another Gungeoneer's past, the Cultist will only have a Dart Gun.
- When using the Gun That Can Kill The Past, the Cultist will become a skeleton during the teleporting animation, just like when the player has a Clown Mask.
- If The Gunslinger dies but The Cultist remains alive, infinite starter gun or Makarov or 38 Special drops will arrive until The Gunslinger is revived or if The Cultist isn't currently in a room with enemies.
- The Cultist can never start the speedrun timer. It may be possible to beat a run in 0 seconds if Player 1 can be killed in the first room.
- Activating Turbo Mode will only apply the effect to Player 1 until Player 1 is killed, in which Player 2 will have the effect.
- The Resourceful Rat ignores The Cultist when stealing an item.
- If either player dies before exiting to the next chamber elevator, the next chamber will have chests that will not turn into revival chests (except for spawned chests). Opening them will cause them to act as a normal revival chest but if both players are alive, they are able to open the unspawned chests without requiring a key.
- If the player enters co-op as The Paradox and uses the alternative gun skin shrine, the Cultist will start without a gun. Entering a room will spawn a supply drop containing a random starter gun or weak gun, which occurs as a failsafe when any character has no usable guns.
- If the Cultist picks up an item that the Resourceful Rat is stealing, the rat will soft-lock in his current position. He will remain in this state, unable to steal dropped items, until the players advance to the next level.
- If the either player dies while Drill is unlocking a chest, the chest will revive the killed player, but will be unable to finish unlocking, trapping the players in the drill room and softlocking the game.
- Nicknames that NPCs refer to The Cultist as include:
- Some guy
- Guy next to the protagonist
- Assistant gun user
- Additional set of hands
- NPCs in The Breach and the Gungeon shopkeepers will refuse to talk to The Cultist, declaring that they will only speak to the protagonist. Some will even insult The Cultist by pointing out their young age or the low quality of their starting weapon (Dart Gun).
- The Cultist's Dart Gun resembles a purple version of the Nerf "Maverick".
- The Light Gun is hinted at originally being the Cultist's; its description states "After a clumsy child of The Gungeon accidentally dropped this toy into a pit, they were forced to amuse themselves with the Dart Gun."
- When selecting the Cultist in The Breach, their icon above their head will spin around, ending in the picture of the Cultist in the icon with their face pressed against the screen.