Tsutsujidai (ツツジ台) is the main background setting of SSSS.GRIDMAN.
Overview[]
A city full of mysteries where Kaiju appear invisible to the populace exempt for Yuta Hibiki for reasons untold. Mysteries continue to gather as the series unfolds, where the city is frequently attacked by monsters created by Akane Shinjo with the help of a mysterious entity named Alexis Kerib.
In Episode 6, Anosillus the 2nd approaches Yuta and introducing herself as a witness of the actual reality surrounding Tsutsujidai, where everything is controlled by Akane being the world's "god" and "deleting" whoever and whatever she disliked, and rewriting events in said of monster attack victims. While the means of undoing the damages are unexplained, the 13 Venoras spread in the city releasing its nerve gasses on the civilians to erase their memories as prevention from learning the truth, putting them in an everyday-life loop where everything is normal through the trains.
Nearing the endgame in Episode 10, Akane's final kaiju decapitates all the Venoras, bringing an end to the fog they emit and its resets and revealing the ceiling boundary above the city revealing to be the Computer World (minorly witnessed in Episode 7).
Everything is explained in the finale, that Tsutsujidai is an inside world within the computer world developed by Akane as a means of escapism from her real-life problems with Alexis Kerib being the catalyst. All its denizens are actually programs called Repli-Compoids (レプリコンポイド, Repurikonpoido) that were made to unconditionally like Akane and believing to be acutal humans. Upon the decisive battle between Gridman and Alexis, Tsutsujidai and the rest of the Computer World were repaired by Gridman's Fixer Beam, healing the damage to become a replica of the real world.