Tsukigasa (a Touhou LW original) refers to an atmospheric optical phenomenon in which a halo appears around the moon. This is an apt temperament for Eirin, who created a fake full moon and sealed the surface. Eirin is endowed with Fire Phase, as she sought change and not constancy, and Moon Essence, as a sage of the moon. The formless and passionate Fire Phase resists Wood Phase, because wood feeds fire, but it is weak to Water Phase, because water dampens fire. The cooperative and indecisive Moon Essence resists the alluring and arrogant Sun Essence, but it is weak to the diverse and uncooperative Star Essence.

The ability of Eirin Yagokoro from the world of eternal night (a Touhou LW original). It is essentially the same as our version of Eirin's ability. She can create any medicine with her amazing skill if she has the right ingredients. She also has an outstanding mind that is not limited to just the field of pharmacy. She is one of the Sages of the Moon, and her intelligence is so great that she was even consulted for advice by Tsukuyomi, who is said to be the founder of the Lunar Capital. It seems that the Eirin of this world is more than capable of prolonging the Eternal Night Incident... While Eirin serves Kaguya, she is also Kaguya's teacher and kind of like her mother or guardian as well. Their relationship is more complicated than a simple partnership. Although Kaguya's abilities are complex, unknown, and expected to be very powerful, it is thought that Eirin has utilized her abilities to create the Hourai Elixir, an immortality medicine. This means that Eirin is at least familiar with Kaguya's abilities and may even completely understand them. It is theorized that Eirin did not just receive Kaguya's help by giving her specific instructions but also performed the extraordinary feat of turning the concept of Kaguya's abilities into an ingredient for medicine. This hypothesis is possible only because Eirin possesses a genius brain and ultimate pharmaceutical skills. And if this hypothesis holds, Eirin should also be able to fully comprehend the abilities of her other students, the Watatsuki sisters, and maybe even materialize their abilities into ingredients for medicine. Unlike our world's Eirin, the Eirin of this world has not managed to come to the conclusion that Gensokyo's barrier alone can sufficiently shut out the moon emissaries. This has directly led to the incident being prolonged, meaning that Eirin's secret technique, which created a sealed chamber by swapping the full moon with a fake moon, was never broken. The eternal night that was created to combat this spell must have also been extended somehow. Perhaps this Eirin's method of creating a sealed chamber was different from what happened in our world. It is unclear how the Eirin of our world performed her spell, but it is possible that this Eirin performed it by manifesting someone else's ability. Thus, it can be assumed that the Eirin of this world also caused the incident to test her spell. The test must have been successful, meaning there was no need to worry about an invasion from the moon emissaries. And even if something unexpected occurred, Eirin could prevent any attack by copying the enemies' abilities as ingredients for medicine, which she could then apply to her spell. Since this would block intrusion more reliably than Gensokyo's barrier, Eirin no longer had any reason to undo her seal on the surface...