(hex:B5) is a glitch item in Pokémon Red and Blue.It is a Rival's effect glitch item which opens the menu from both in and outside of battle. From outside of battle, if the player exits the menu with B they can enter a glitched state where they can walk through walls that are one tile thick.
If the player selects a Pokémon without full HP instead, it will heal up to 80 HP and use up the item.