Chikyuu-kuudou-setsu (a Touhou LW original) refers to the theory that another world exists beneath the surface of a hollow planet Earth. The Underworld is made from the memories of the Earth, and the deeper you delve, the older the memories you will find. The planet Earth is like a colossal brain, and the Underworld contains the more primal and bestial components. This includes things like dreams, the collective unconscious, and delusions, which all intertwine in a complex array. Because of this Koishi's peculiar circumstances, her Phase and Essence endowments are entirely different from the usual. Koishi is a being in the form of an optical illusion shown by the unconscious mind, so she is resistant to Sun Essence (surface laws), Moon Essence (surface changes), and Star Essence (surface freedoms), as the three celestial Essences are all surface-level perceptions. However, she appears to be weak against Fire Phase because it is the element of intense emotion.

The ability of Koishi Komeiji from the buried city (a Touhou LW original). This Koishi Komeiji was a world-hopping ally of the blue Reimu. However, a mistake occurred when she tried to cross between worlds, which resulted in her becoming the first youkai to ever exist in the Underworld that is destined never to connect with the surface. The plant-like thing that accompanies her is the remnant of the multi-dimensional neural network... an organism that has undergone Convergent Rebirth. This being has not retained its original form, but it acts in pretty much the same manner as blue Reimu's divine spirit, which is the spitting image of Suika... There is no way to determine what this being's original form was since even Koishi herself does not remember. There is not even a clear definition of what exactly the multi-dimensional neural network was or is. If only there was a way to figure out what kind of Satellite Koishi brought with her before she ended up like she is now. That would enable further research into the nature of the Satellites and the Omni-Incident Organism... But the world-hoppers don't like to talk about their Satellites, which is why the blue Reimu and the other world-hoppers don't know much about Koishi's Satellite even though they used to work together. The Satellites could potentially be the world-hoppers' greatest weakness. That is why the Spell Cards that utilize these Satellites are always Last Words. There is no doubt that the central figure of the world-hoppers is that tiny Yukari Yakumo. But somehow that Yukari Yakumo was forced into a corner. The world-hoppers hurried to perform an emergency escape, but some of them didn't manage to escape successfully... They were impeded by someone or something. This Koishi was one of those world-hoppers. Koishi should have been able to escape unnoticed by everyone, so who or what could have possibly caught her? Regardless, this resulted in Koishi getting pulled into a vortex that transcended time, space, possibilities, and even the gaps between dimensions. The only consolation was that she happened to awaken in a world where the fundamental laws and format of history were almost the same as the world she left. However, that is where this Koishi Komeiji's luck ran out. She awoke firmly trapped within the center of Earth's crust, right in the middle of a stone, during the beginning of the Age of the Gods. There still wasn't even a single hint of any intelligent life on the surface, and the primeval cells were just about to begin their bitter fight for survival. Koishi had expended all of her magic, and there was little hope of her recovering any in this age where the concept of mysteries hadn't even been born. She would have to wait hundreds of millions of years to escape her prison. She couldn't possibly endure this if she wasn't an unconscious youkai. However, youkai cannot survive unless someone fears them... so does that mean someone feared Koishi? The multitudes of different primeval lifeforms were the ones who supported Koishi's existence. They were mere sparks without a single conscious thought that would get extinguished by the slightest breeze. However, the network of microscopic lifeforms harbored a vague fear of death. This was different from the unconscious mind from which she originally drew her power, but her altered Satellite... the multi-dimensional neural network, saved her. Time flowed. And as the years rolled by, various things fell down to the Underworld―the tools of gods that were no longer used after the Age of the Gods had concluded, fossils of extinct creatures, strata records created by the planet... and sealed youkai. Even the memory of an unconscious youkai fades over time, so she should have long since forgotten the face of her older sister, whom she earnestly sought... but she did not.