The Rival's effect (also known as Jack) is an effect in Pokémon Red, Green, Blue, and Yellow, caused by certain glitch items that allows the player to temporarily become invisible and move through normally impassable tiles that are one or two tiles thick.
In Yellow, it is also capable of desynchronizing the walking Pikachu so that is more than one step away from the player, because Pikachu will stay when the effect is active; which is useful for the Pikawalk glitch (Pikachu off-screen glitch).
The glitch is known in Japanese as "missing wall" (Japanese: 壁抜け).
The glitch gets its English names because "RIVAL's" (hex:A2; Pokémon Red and Blue) and "JACK" (hex: B4; Pokémon Blue) are two of the glitch items that can activate the glitch.
Performing the glitchThe Rival's effect is activated by using a "Rival's effect item", which can be identified by adding +80 the the hexadecimal ID of an item that opens the Pokémon menu. For example, the glitch items "E tE" (hex:94) (Red/Blue) or "o" (hex:94) (Yellow) function like a Potion (hex:14) and are examples.
Any glitch item can be obtained in the English versions of Pokémon Red, Blue and Yellow using the Celadon looping map trick. In the Japanese versions, another item creation glitch must be used; due to the x-position address corresponding to a quantity and not an item.
If the item is an unterminated name glitch item, then a hex:50 tile (such as the bottom-left corner of a bush) must exist somewhere on the screen to avoid corruption after selecting it (but not using it).
List of Rival's effect glitch itemsSee Category:Rival's effect glitch items
StepsFace a passable tile. Use the Rival's effect item to open the Pokémon menu. Close the menus with B. The Pokémon sprites from the Pokémon menu will still show, and the player and NPCs will disappear. Try to move in the direction you want to go, until you become stuck. You may be able to pass through a tile even if it is normally impassable. Open the Pokémon menu from the start menu and close it to bring the sprites back and be able to walk again.
CuriositiesMoving in front of the locked Viridian City door while under the Rival's effect will cause Red to walk down and jump down a ledge (it may normally be impossible to make him jump by moving to another ledge). Red's shadow will show but not Red himself. If you open the menu while your shadow is showing, the shadow will be replaced with 9 and a horizontally flipped 9 in Pokémon Yellow and 9, the flipped 9 and both the 9, flipped 9 flipped vertically in Pokémon Red and Blue.
Using a valid item that opens the menu (such as a Rare Candy) after closing the Pokémon menu the Rival's effect glitch item brought up may make Red's sprite appear over the menu. Closing the start menus will activate the Rival's effect with Red visible on the screen (normally Red is invisible during the Rival's effect glitch). Red may be able to move facing the direction he was without turning or his sprite animating.
If you open the Pokédex or the Town Map immediately after using the Rival's effect item, the Pokémon menu sprites will still appear on the menu. Pressing up may make the game register it as a down press the first time. (documented by drc and Torchickens).
ExplanationCertain items are part of a list of items that open the Pokémon menu. Rival's effect glitch items open the Pokémon menu, but are not included on the list; meaning that when the menu is closed part of the game manages the player as if the party screen was still open. (explanation by TheZZAZZGlitch)
These items include: