From Bulbapedia, the community-driven Pokémon encyclopedia.
The Key System menu
The Key System (Japanese: キーシステム Key System) is a feature of the Unova Link in Pokémon Black 2 and White 2. It allows players to change the difficulty level of the game, switch between Black City and White Forest, and change the Mystery Door in the Underground Ruins. They can be shared between players using the Unova Link, but players can only share the keys that are exclusive to their version.
Difficulty
There are three different difficulties: Easy Mode (Japanese: アシストモード Assist Mode), Normal Mode (Japanese: ノーマルモード Normal Mode), and Challenge Mode (Japanese: チャレンジモード Challenge Mode). Only Normal Mode is unlocked by default; Easy Mode and Challenge Mode are exclusive to White 2 and Black 2 respectively, and are unlocked by defeating the Champion. This awards the player with the Easy Key (Japanese: アシストキー Assist Key) in White 2 or the Challenge Key (Japanese: チャレンジキー Challenge Key) in Black 2.
Because keys are tied to the save file, starting a new game or deleting the save file will also remove any unlocked keys. Therefore, the only way to play the main storyline with a difficulty other than Normal Mode is to transfer another player's key from a separate DS/3DS system.
In Easy Mode, opponents use less competent artificial intelligence and the level of opposing Trainers' Pokémon are lower than usual. However, their Pokémon's stats are still calculated using the levels from Normal Mode (for example, Champion Iris's Haxorus will appear to be level 55 but still have the stats of a level 59 Haxorus). The lowered levels are taken into account for prize money (the player will receive less), but not experience (in the example, the player's Pokémon will still receive experience as if they had defeated a level 59 Haxorus).
In Challenge Mode, opponents use more competent artificial intelligence and the level of opposing Trainers' Pokémon are higher than usual (1 level higher at the start of the game, and rising to 5 levels higher by the end of the game). Gym Leaders and the Elite Four each use one additional Pokémon on their team, as well as their party having different movesets and held items, and their Pokémon's IVs are all set to 30. However, Trainers' Pokémon's stats are still calculated using the levels from Normal Mode (for example, Champion Iris's Haxorus will appear to be level 63 but still have the stats of a level 59 Haxorus). The raised levels are taken into account for both prize money and experience (the player and their Pokémon will receive more of each).
City
The version exclusive cities, Black City and White Forest, are found by default in Black Version 2 and White Version 2, respectively. They can be unlocked in the opposite game if certain conditions are met.
After completing five areas of Black Tower in Black 2, the player will be rewarded with the Tower Key (Japanese: まてんろうのカギ Skyscraper Key). This can be transferred to White 2, which will unlock access to Black City. Likewise, after White 2 players have completed five areas of White Treehollow, the Treehollow Key (Japanese: じゅどうのカギ Tree Hollow Key) can be transferred to Black 2. This unlocks White Forest for Black 2 players.
Players of either version can use the Unova Link to switch between cities once both are unlocked. However, the cities cannot be switched if the player had saved their game within the current city.
Mystery Door
A mysterious door in the Clay Tunnel gives access to the Underground Ruins, where the legendary giants can be found, though only one giant will appear at a time, depending on which key is active. Regirock is the default Pokémon in both Black 2 and White 2. Upon catching Regirock (and not just defeating it, fleeing from it or obtaining it from another game), Black 2 players are rewarded with the Iron Key (Japanese: くろがねのカギ Iron Key) and White 2 players with the Iceberg Key (Japanese: ひょうざんのカギ Iceberg Key); these allow access to Registeel and Regice, respectively. Players can obtain their final key by transferring it from the opposite version through Unova Link.
Trivia
- It is possible to receive a transferred key on a game that does not have a save file yet. If the player then starts a new game, they will keep the key. If the player then quits the new game without ever saving and starts another new game, they will still keep the key. However, if the player saves, then either removing the save file or starting a new game will remove the key.[1]
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