Mega Stone
Can be used to Mega Evolve any compatible partner Pokemon in battle.
For example, if you have a Pikachu and a Charizard out, you can give Pikachu a Charizard X mega stone, and give Charizard Leftovers. When the Mega evo is activated, Charizard evolves in to Charizard X and is able to continue to hold Leftovers.
If your Pokemon is holding a Mega Stone compatible with an opponent Pokemon, it can be used to Mega Evolve that opponent. In the event that both players have the same species of Pokemon, the users Pokemon gets priority.
Fish Hook
Activates upon holder making physical contact with an enemy. The enemy cannot switch out and is dealt 1/16th damage at the end of each turn. User is unable to switch as well. Hook can only be used once, and breaks when either Pokemon faints.
Shovel
All physical moves function like Dig. The first turn, the user burrows underground. The second turn, the user lands its attack.
Communication Headset
When the Pokemon holding this item faints, you can select a different fainted Pokemon in your party to restore 20 HP and send out.
Spicy Leftovers
The Power of all of the users special attacks increases by 10 at the end of each turn, up to 50. Resets when the user switches out.
Vitamin
The Pokemon that holds this item restores 1 PP to every move at the end of each turn.
Incriminating Evidence
Looks like an open manila folder with some document inside.
Lowers the holders HP by 1/8th and PP of every move at the end of each turn. If transferred from one Pokemon to another during battle, the Pokemon that received the item that turn has their move skipped the following turn.
