The Barcode Boy is a Game Boy peripheral developed by Namco and released only in Japan in December 1992. It attaches to the Game Boy and is used to scan or swipe barcodes, similar to the e-Reader.
Games such as Monster Maker: Barcode Saga were designed to be used specifically with the Barcode Boy. Scanning the game cards would give the player the ability to custmize characters and battle opponents via a link cable.
Other barcodes found on actual store-bought merchandise could be scanned as well, giving the player different features in the games.