Character ID

Character ID is the number attached to a character on an account, relative to the total amount of created characters across all accounts. Some Pokémon and items are available to a player based on the ID number that they have. They are exclusive to said character, and thus the player can only encounter that specific Pokémon or item at that location. As such, the only way to obtain the opposite ID Pokémon and items is with or through another player whose ID is tied to them.

Spawn rates are independent and will not be influenced by the number of players in a party. The ID on a character cannot selected when creating nor be changed. Those who wish to have a different ID must create a new character.

ID Pokémon
ID Pokémon appear in specific dungeons. The respective Pokémon and dungeons are as follows:
 * Plusle/Minun appear in Thunderstorm Forest from Floor 11 to 20.
 * Seviper/Zangoose appear in Winden Forest from Floor 11 to 18 and Deep Winden Forest on all floors.
 * Spritzee/Swirlix appears in Daydream Dream Grove in the Royal Garden (through Honey).
 * Skrelp/Inkay appear in Nightmare Dream Grove in the Royal Garden (through Honey).
 * Skrelp/Clauncher appear in Sandy Shallows from Floors 21 to 30 (Dawn/Day and Dusk/Night only, respectively).

There's also the Generation 5 starters Snivy, Tepig, and Oshawott. They have Mystery Eggs on Floors 21, 31, 41, 51, 61, 71, 81, and 91 in Harmonic Tower, which can spawn based on the IDs of players entering the floors. They are the only Pokémon assigned differently regardless of Minun ID or Plusle ID, since they make up a trio.

ID Items
There also exists more uncommon ID items. These items appear based on the character ID and spawn randomly in dungeons, much like ID Pokémon.
 * Shoal Shell and Shoal Salt appear in Southern Sea; Plusle IDs will see Shoal Shells, while Minun IDs will see Shoal Salt. However, these two items appear regardless of ID in Beach Bunker and Sandy Shallows

Undercity Gangs
Character ID also indicates which Tanren Undercity gang they belong to. Plusle IDs are in Desert Fangs, and Minun IDs are in Night Talons. Each base has items able to be bought that are not in the other.
 * The Rocky Helmet, Assault Vest, Choice Band, and Team Skull music items are exclusive to the shop in Desert Fangs' base.
 * The Weakness Policy, Safety Goggles, Choice Specs, and Aether Foundation music items are exclusive to the shop in Night Talons' base.

Determining ID
Players can use the "/glomp" command to see if their account will spawn Minun or Plusle. This command is only gives results for these two specific Pokémon, but can be used to determine whether the player will encounter the other ID Pokémon. For example, If the player sees Plusle, they will also see Seviper. If the player sees Minun, they will also see Zangoose.

For a more accurate description, players can also use the "/charid" command. This tells the player the complete set of ID Pokémon they can get, as well as if they belong to the Night Talons or Desert Fangs, and the player's actual character ID. /charid however will not say whether a player can see Shoal Shell or Shoal Salt.

Trivia

 * Mareanie and Frillish were previous ID based Pokémon, appearing in Sandy Shallows in place of Clauncher and Inkay.