Players in the United States that pre-order
Pokémon Conquest at a participating Gamestop location by Sunday, June 17th 2012 and pick it up by Tuesday, June 26th, 2012 will to get five exclusive passcodes to unlock the in-game characters: Lapras, Emolga, Riolu, Scyther and Pikachu
Online Customers: Code will be emailed within 2 business days after your order ships. PowerUp Rewards Card must be entered during checkout.
Store / In-Store Pickup Customers: Code will be printed on your receipt at time of pickup. PowerUp Rewards Card must be scanned at checkout.
Source:
http://www.gamestop.com/ds/games/pokemon-conquest/100555#bonusFeatures