Lunarian pharmaceutical genius responsible for creating the Hourai Elixir that gave her good friend Houraisan Kaguya - and later Kaguya's bitter nemesis, Fujiwara no Mokou - immortality. She has since been Kaguya's bodyguard and caretaker in their mansion, Eientei. She is also the mentor of Kaguya's pet lunar rabbit, Reisen Udongein Inaba, and by extension Reisen's charge, earth-born rabbit youkai Inaba Tewi. Owing to her expertise, she became Gensokyo's chief pharmacist.
Due to her estimated age (well over 1000 years old and possibly one of the original Lunarians) and her relatively mature demeanor (compared to her charge), she is often included in the "Old Maids Alliance" with Yakumo Yukari and Yasaka Kanako.
Most famous for the fans' rallying cry "Eirin! Eirin! Tasukete Eirin!" which, in fact, originated from Houraisan Kaguya.
On her hat is typically the red cross symbol. At the bottom of her dress are trigrams composing the bagua. She is frequently depicted with constellations sewn into her dress.
Her surname invokes the name of the Shinto god of intellect and wisdom, Yagokoro-no-Omoikane. This, combined with ZUN's comment about her being in "the higher order of gods", gave rise to the belief that she may in fact be Omoikane.
See also
"Yagokoro", a horribly failed attempt to draw her on Shift_JIS
