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Dr. Mason will offer you one of three decks:
The Charmander Deck uses Fire, Lightning, and Fighting type Pokémon. This is the right deck for you if you find evading all effects of attacks and making all the Energies attached to a Pokémon turn to the only type you need very useful.
The Squirtle Deck uses Water, Fighting, and Psychic type Pokémon. This deck lets you attach as many energies as you like during one turn to any of your Pokémon and half the amount of HP the opponent has.
The Bulbasaur Deck uses Grass-type and Water-type Pokémon, and a Fire-type too. With this deck, you can transfer energies and use an attack that can normally give up to 80 damage to the opponent.
Now that you've chosen, let's review the menu. It will be opened with "Start" and closed with "B". There are 6 things to do with the Start menu:
"Status," which shows this:
||<Number of different kinds of cards collected>/226
||<Number of hours>:<Number of minutes>
"Diary," which saves the game,
"Deck," which lets you modify your decks, change their names, choose which one to use in duels, and create new decks.* Press "B" to exit and use the Deck menu: Confirm to see the cards in the deck, Modify to keep modifying the deck, Save to save the deck, Dismantle to break the deck up into the 60 cards it contains, Name to change the name of the deck, and Cancel to stop modifying the deck and not save changes.
Next, "Card," to view all your cards,
"Config," to change your settings: Text Speed and Duel Animation. You probably will want 5 in Text Speed. The options for Duel Animation are Show All, Skip Some, and None. To save, use Exit Settings.
Last is "Exit". That will exit the Start menu.
Another tip: Press "B" while you're walking and you'll run. The PCs are helpful too, with Mail. All mail is from Dr. Mason, who has sent a booster pack.
In the other room, you can fight against Tech Aaron, and you can choose the kind of deck he uses, but the prize booster pack of Colosseum that only has Energy cards.
In the northwest corner of the room, there's a deck builder, although you don't have the cards to build any of the five decks: the Charmander & Friends Deck, the Squirtle & Friends Deck, the Bulbasaur & Friends Deck, the Psychic Machamp Deck, and the Water Beetle Deck.