Underworld is set in a past era, when science and philosophy were just being discovered, but instead of recreating a past era, limited terrain and certain customs were introduced so that the inhabitants of the world would form their own society. For this reason, a virtual world, codenamed «Underworld», was created using the free kernel version of the Cardinal System, The Seed, and then transferred into the STL mainframe via the Mnemonic Visuals feature of the STL. By using a 4th generation FullDive machine, called the Soul Translator (STL), Rath succeeded in copying the Fluctlights (souls) of twelve newborn babies, creating a «Soul Archetype», a non-developed Artificial Fluctlight that could be copied to create new Artificial Fluctlights, which would then need to be nurtured to mature into an individual. Project Alicization, a top-secret government project run by Rath, a company affiliated with the Japan Self Defense Force (JSDF), was started to create the first highly adaptive, «Bottom-up AI». Other prominent characters from the franchise such as Asuna, Leafa, Sinon, Lisbeth and Silica also feature. These conversations with Twitter users would dictate her personality over time and ultimately decide if she would become a 'good girl' or 'bad girl' in-game. Although she is part of the game's initial release, a later patch would tweak her 'vocabulary and personality' in-game to a 'specific character', based on keywords in Twitter tweets as part of a promotional campaign featuring the character.

The main character of the game is a 14-year old AI, known as Premiere, that travels with Kirito.

However, unlike in Sword Art Online, Ainground is one large floor, instead of multiple, while the castle itself was renamed to Aincrad, where several monsters can be battled. Es mejor conocido como el creador de Sword Art Online y Accel World, ambos adaptados al anime. Hollow Realization takes place in Sword Art: Origin, a restoration of Aincrad titled Ainground, with intentions of research and development in the year 2026, three years after the original Sword Art Online event. Reki Kawahara (, Kawahara Reki, nacido el 17 de agosto de 1974) es un autor japonés de novelas ligeras.

Kawahara reki