The Barcode Boy is a Game Boy peripheral 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.