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Eirin Yagokoro is a genius of medicine from the Moon, able to create even the Hourai Elixir. She is living in exile in Gensokyo after choosing to stay with and devote herself to Kaguya Houraisan on Earth rather than take her back to the Moon. She was a very well-known figure on the Moon, having not only been instrumental in the founding of Lunar society with Lord Tsukuyomi, but also being its leading medical mind and the person in charge of the Moon's emissaries to the Earth.
It seems that now Eirin has begun using her knowledge of medicine to act as Gensokyo's premier pharmacist, selling medicines at Eientei ranging from sleep aids to various cures. She is generally a fairly pleasant person and is devoted to serving her best friend, Kaguya, but she does not suffer fools gladly, and because of her incredibly high intelligence, she can come across as conceited, impatient, and condescending at times to those not sharp enough to follow her explanations immediately.
During the events of Imperishable Night, Eirin attempted to seal the passage between the Earth and Moon, which becomes open during full moons, by casting a spell that would seal the Earth inside a chamber (similar to a Chinese folktale about a paradise found inside a pot). She only meant to do this to protect both Kaguya and Reisen so that the emissaries coming from the Moon wouldn't be able to reach them, but this did not go unnoticed by the various Youkai in Gensokyo who enjoy the moonlight.
During Silent Sinner in Blue, Eirin spied on the humans to see what they were up to. She requested the Watatsuki sisters to train harder to be able to battle the people from earth. The two sisters successfully defeated the invaders.