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There are many situations in these three games where an NPC wishes to accompany the player while in a particular area. Riley, for example, travels with the player while on Iron Island. In such situations, the NPC (the partner) will follow the player on the overworld. The player will not be allowed to use most HMs (such as Surf) or the Bicycle. All battles—be they wild or trainer battles—become double battles, with the partner's AI-controlled Pokémon fighting alongside the player's.
Using a walk-through-walls cheat while accompanied by a partner will corrupt that partner. They will turn invisible on the overworld, but the player's movement will still be limited. In wild battles taking place on water, they may use the player's name and back sprite and usually send out DPBox. Wild battles taking place in grass may become Safari Zone double battles, with both the player and the corrupted partner automatically throwing Safari Balls. (The partner's Ball will simply spin in midair; the player's ball will capture the target, but the game will freeze upon attempting to show the target's Pokédex data.)
In some cases, glitched partners may spontaneously vanish, only to reappear later in certain areas. Some may appear after a player walks through walls, even if they have not previously glitched a partner. In one case, a player used a walk through walls cheat; upon entering Oreburgh City, Marley spontaneously appeared as a partner.