Mugetsu (a Touhou LW original) refers to the Moon hidden by rain or clouds. Eirin's head-mounted display, which looks like a blindfold, prevents her from seeing the real world. Her temperament is not limited to the usual archetypes because the colossal system of the Capital is altering it. This temperament is specialized to solely resist Earth Phase.

The ability of Eirin Yagokoro (B3) from Multiverse Trail 6 (a Touhou LW original). This Eirin's universe is irregular. Due to how the Multiverse Trails work, it shouldn't be possible for Isotropic Beings (different versions of the same individual) to exist in the same universe. The ecumenical niche will not allow it. Even if there might be a brief window where nothing bad happens, the chance of this state being able to last for a significant length of time is astronomically low. However, the Capital's Senior Sisters System (SSS) makes this possible. The Senior Sisters enable Isotropic Beings to exist in the same universe, but they can only exist in the same universe because this is possible. Nully seems to be the key to solving this paradox. Multiverse Trails are slightly different paths of fate followed by variations of what should have been identical beings. So, if natural means of interworld travel are used, multiples of the same individual from different Multiverse Trails should never exist in the same universe. The chances of that happening are infinitesimally small. It would be like if you took apart a sacred pyramid, threw the blocks in the air, and they all just happened to land in the shape of a pyramid again. It shouldn't happen, no matter how many times the universe repeats from beginning to end. Not to mention that this incredibly low probability decreases exponentially with each Eirin added to the SSS. The only possible explanation is that they used a highly advanced artificial means of interworld travel. In most universes, the Lunar Capital possesses a level of technology that can only be called super-science, but this goes way beyond that. Only Eirin Yagokoro's genius mind could ever make this a reality, and the more Eirin minds that work together, the greater the chance of this artificial means succeeding. No matter how many Eirins are gathered together, their combined intelligence is always more than enough to offset the probability decrease caused by having multiple identical beings in the same universe. That is how the SSS sustains itself. Null Character was summoned to deal with the ecumenical niche's repulsive force, so she was probably there during the initial stages of the SSS's inception. However, for some unknown reason, she is numbered as the last individual entity. The following is a report written by a former member of the SSS who ended up in another universe. "Are you really going?" I was facing away from the Capital. The Toyohime speaking to me would soon become just another parallel world Toyohime, and my current life would have no more meaning than the countless other lives lived by me and my other selves. That's what it means to travel between worlds. "Yes! I've made up my mind. I have long since finished what I was brought here to do, so the Capital will be fine without me." I'm wise enough to know that this didn't answer what Toyohime really wanted to ask, but there was nothing else I could say. "Even if you're just a single part of the colossal brain made up of multiple Lady Yagokoros, to me, you're still Lady Yagokoro. I love and respect you. Are you truly going? Is there nothing I can say to stop you?" I wasn't planning to dignify the same question with a response, but I couldn't help but wonder why she was trying so hard to stop me. The reason she gave was insufficient. Sure, she may respect me, but there were plenty of other Eirin Yagokoros there who she also loves and respects. "Tell me the real reason. Why do you want to stop me from leaving?" "You're not defective." That was all she needed to say for me to understand, but she continued. "When Yorihime summons divine spirits, she can only ever summon bunrei, a portion of the god's divine spirit. Bunrei are said to be the equivalent of the original god, but that only applies to their authority. They only possess a sliver of their full data." In the past, when the Capital was trying to figure out how to turn the central regions into a brain, an idea was put forward to summon material caretakers of the pyramid facility from parallel worlds to use as materials. However, the major flaw with this plan, and the reason why it was scrapped, was that the results could never surpass expectations. The caretakers of the pyramid facility would all have to be within range of Yorihime's summoning, and Yorihime's ability is limited by her imagination. This wouldn't resolve the main issue. "That's why Yorihime had to summon you first―a Lady Yagokoro with a fragmented fate. It wouldn't have succeeded otherwise. You're just too intelligent and wise..." As her voice trailed off weakly, I almost began to question whether there was really such a big difference between us. She's a fellow sage and my disciple. But, no. I knew that wasn't what she meant. "You're not defective. We simply weren't strong enough." "I know." Wishing that she wouldn't cry over this, I doubted my decision. I did what I never thought I would. I turned around. It meant admitting that I had made a mistake and redoing something I had already started. Perhaps it was only a minor thing, but I had never once thought that I might have been wrong before. "I'm sorry... Thank you, Toyohime." I walked back to Toyohime, who had her head in her hands, and hugged her. I'm small, so I had to reach up and stand on my tippy-toes. But that was my only concession. "All right. I'm going now." Turning back around, I faced away from the Capital. I would not be returning. As long as I stayed there, my beloved disciples would continue to curse and lament their own weakness.