From Glitch City Laboratories
This article refers to the error handler within the first generation of Pokémon games used to refer to the player's name if he or she was not already named. For the company, Nintendo, see Wikipedia's respective article - Nintendo
The dialogue string NINTEN is a factory default and error handler used in the English versions of Pokémon Red, Pokémon Blue and Pokémon Yellow. It used as an exception for when the game attempts to manage a save file, but information about the name of the player character is undefined. If the player has not been named, the game will use the term "NINTEN" instead when attempting to display the name of the player. In normal gameplay, the player has to name his or herself before starting a New Game and arriving in his or her room, therefore the word 'NINTEN' is normally never seen, without the use of a game-altering device. The name is presumably shortened from the name NINTENDO, as supported by the fact that the default string used to refer to the player's rival is also (perhaps as part of an in game joke) 'SONY' who often were and often still are considered as one of Nintendo's main rivals. The name was presumably shortened to 'NINTEN' because only seven bytes were used to define the player's name (letter by letter), meaning that names could only be a maximum of six letters long (as the seventh byte always should be the hidden 'end character' letter).
By using a cheating device or game-enhancing device such as the GameShark, players can enter the code 010288D0 (010287D0 on Pokémon Yellow) to replace the first option on the menu following the title screen with 'Continue'. If the player does not have an existing save file, the game will give the player's name as 'NINTEN'.