Neptune is the protagonist of Hyperdimension Neptunia. She is named after the aborted Sega Neptune game console. The developer Compile Heart licensed the name from Sega, making Neptune the only console-tan in the game to have some sort of manufacturer approval.
Playing on the fact that Sega is absent from the current generation of consoles, Neptune isn't a goddess like the rest of the console-tans at the start of the game but first appears as a girl that has lost her memory.
The Sega Neptune was a proposed console that combined the Sega Genesis with its add-on, the 32X, making the two devices a single unit. However, the project was scrapped after it was determined that there wasn't a market for the device. The Neptune never moved beyond the prototype hardware stage.