- "Now listen up. This Shibuya ain't your Shibuya. It's the Composer's Shibuya, which the Reapers use to run the Game. No matter what happens, people won't see you, won't hear you-- and they definitely won't help you. Your partner's the only one who can keep you alive."
The Underground, abbreviated as UG, is a parallel plane of existence of stacked above the Realground (RG) used to conduct the Reapers' Game. It is created and ruled by a Composer, and overseen by a Producer from the Higher Plane. Only people tuned into the UG or the Higher Plane, like Reapers, Players of the Game, and Angels can see. The UG is stacked below the Higher Plane and is highly tuned into the Noise plane that it sandwiches with the RG.
Hanekoma explains in the Secret Reports that while Players typically come to the UG after death, they are not dead but rather forcibly tuned into the vibe frequency of the UG. Due to the UG's higher vibration frequency, those in the RG cannot generally see, hear, or in any way perceive events and people in the UG, while those in the UG can perceive RG events freely (though physical interaction remains difficult). Certain people are able to see the UG or the Noise plane from the RG, what causes this being unclear. Joshua is the most prominent example, though certain random NPCs have Thought Fragments indicating they are at least able to perceive the existence of Noise symbols or even Reaper wings while still living in the RG. Certain designs can connect the UG and RG, permitting Players to be seen by and interact with RG people and objects. The designs on a Reaper Creeper panel are an example of this, as are the emblems which permit Players to enter and use shops. The Reaper decals on Reaper-approved shops forcibly tune Players into the RG.
Stray Noise created from people's negative emotions in the Realground are common in the UG, and can be fought by Players through Scanning. When a Player fights a Noise, they are spirited away and forcibly tuned into the Noise plane. To return to the UG they need to win the fight.
In Week 2, Day 4, Joshua states that there are many UGs aside from Shibuya's, each one with its own set of rules. He compares this to how each human being's mind is separate and operates by its own set of rules. NEO: The World Ends with You, taking three years after The World Ends with You and the Shinjuku Inversion shows Shibuya's UG controlled by Shinjuku Reapers and the rule changes that follow.