Halley's Hollow Earth (a Touhou LW original) refers to the theory proposed by Edmond Halley in the late 17th century. This is an apt temperament for Koishi, the white brain cell, from the world delirious with New Year's celebrations. She has retained the same temperament. (Strong against Sun Essence and Moon Essence, weak to Fire Phase.)

The ability of Koishi Komeiji, the white brain cell, from the world delirious with New Year's celebrations (a Touhou LW original). Her ability doesn't seem all that different from the F1 Universe Koishi's. This world sure gets a lot of parallel-world visitors... A parallel world investigation order came down from Yukari Yakumo. The mission was to survey a world that had been swept up in non-stop New Year's cheer. But this Koishi was given a different directive: to visit home for a change. "Strange. I thought this world, where all the interworld travelers gather, was my home. Then again, if we're talking about the place I spent the most time, then home would be the ancient Earth's core. Or does the world where I was born count as home? The world where I began my journey as an interworld traveler. Haha, just kidding. I know exactly which world you're talking about." They may call Koishi "the white brain cell," but she probably doesn't have a spoonful of actual brain matter. Most likely, no youkai have brains. Or if they do, they don't use them to do their thinking. So how do they behave intelligently? Because their entire body acts as their brain. From the human scientific perspective, the total function of the brainstem and nervous system is referred to as the mind. This means that youkai, as spiritual lifeforms, could be considered beings made entirely of brain matter. (There are animals in the human world said to be like that, after all.) Temperament is not merely some fantasy concept. It's a high-order particle with a variety of characteristics. If a single particle of temperament can house a memory or ability, then it stands to reason that a youkai's self can be defined at the particle level. (The people of Hourai may be the ultimate example of this.) But if that's the case, then what is a self that exists as a cluster of temperament? This gives rise to problems regarding self-identity. Are your limbs you? Is your clothing you? Why, in the first place, is the self linked to the size of your body? Why can't it be said to reside in the temperament particles that make up your body, or in all the temperament particles composing the galaxy? For humans, these are profoundly difficult questions. But for youkai, perhaps the act of asking them seems nonsensical. It's possible that youkai can adjust the size of their own self―Suika Ibuki being a prime example of this. Or maybe their self takes different sizes simultaneously. They say if you dissect a planarian flatworm, each piece retains all of its abilities and experiences, serving as proof that the soul can be divided. (Suika and youkai multiplying Spell Cards follow this logic.) If souls can be divided, surely that means they can be merged. Your heart is you. A single cell is you. The clothes you wear are you. The house you live in is you. Even a little rock sitting on the side of the road could be you. The ability of Koishi, the white brain cell, is certainly useful when it comes to interworld travel. Any Koishi, no matter where you bump into them, is naturally suited to leaping between worlds. "Feels just like home," said Koishi, resting in the Palace of the Earth Spirits of a distant world. She laid out the photos she'd taken atop the carpet. Then, the others helped her file them into an album as they chatted away happily. How she wished she could stay forever. Suddenly, she heard a whisper. "You're free to stay as long as you like." She turned to Satori, but her lips were pressed shut. A glint in her eye, she scanned the floor for the next photo to add to the album. Maybe Koishi had imagined the whisper...