Mokou still detests Kaguya, and Kaguya still tries to kill her, but neither of them can die forever. These are wonderful days... This broken place is truly a paradise!
And she is alive to enjoy it, how magnificent!
~ The text of Imperishable Night
Ghosts who have forgotten nirvana cannot be reborn. A human who cannot die will not know the vivid Netherworld. As one really lives, lives, lives, she doesn't know the start of life. As one really dies, dies, dies, she doesn't know the end or death. As I do not know death, I have surpassed that darkness. See this beautiful danmaku, unchained from the dark samsara!
~ Fujiwara no Mokou
The Hourai Elixir is a forbidden medicine that should never be consumed by humans. With a single touch, you will never become an adult. With a second touch, you will forget all disease and illness. With a third touch... Your souls will forever churn in torment!
~ Fujiwara no Mokou
Fujiwara no Mokou is a hero and one of the characters from the Touhou Project series who first appeared as the Extra Boss of Imperishable Night.
She is the longtime archenemy and rival of Lunarian princess, Kaguya Houraisan, whom she has a vengeance towards, although through the progress of thousands of years and the series, she has subsequently grown much less vengeful, but nonetheless enjoys fighting with her.
Fujiwara no Mokou was formerly an ordinary human, but she became an immortal being after drinking the Hourai Elixir about 1300 years ago. Now she is never able to die, though she still feels the pain of injuries as normal. She's considered to be very powerful, given her long life, the power she earned through her life, and her immortal status since she can keep fighting until she can no longer stand the pain.
She has hated Kaguya ever since the moon princess humiliated Mokou's father by turning down his marriage proposal after he failed her trial of the Impossible Requests. Even though it has been over a thousand years since that happened, she continues to detest Kaguya, knowing full well that she can never get her revenge—nor can Kaguya ever put a permanent end to her. When not fighting with Kaguya, though, she aids the nearby villagers by exterminating youkai and will even guide them safely to Eientei for medical treatment. She is a loner by nature and chooses not to socialize with humans or youkai and has a dubious personality that can easily switch from friendly to threatening and back again, but her actions seem to indicate she may be a good person.
(Touhou 8) Red eyes and ankle-length ivory hair. She wears a light brown shirt that appears as if it has been discolored from fire, and dark red overalls that are randomly decorated with paper charms. Her hair is tied with these same red and white paper charms.
(Touhou 13.5) Same as above, except her hair is waist length, and her shirt has short sleeves.
Mokou is described as being a quiet person, and doesn't generally enjoy the company of others. However, she was easily roused during Imperishable Night's extra stage, where she goes on a rant about how annoying Kaguya is. Not much else is known about her as she doesn't really talk much to anyone. As a powerful human (or a being who was once human), one of her main jobs is youkai extermination, though she seems to be settling on guiding visitors to Eientei in and out of the Bamboo Forest of the Lost.
In Cage in Lunatic Runagate, she is relieved that Kaguya appears to be staying on earth rather than returning to the moon, as the two of them are in the same mess of being immortal beings. Mokou also notes that one day she wants to travel to the top of the Youkai Mountain, presumably to see Iwanagahime, the goddess of immortality, and to apologize to her for what she did to Iwakasa.
(Note: The Youkai Mountain was once the mountain Yatsugatake, which is the mountain Sakuyahime suggested to Mokou and Iwakasa to bring the elixir to instead.)
The characters for Fujiwara literally mean "Wisteria field", and Mokou "Scarlet (younger) Sister". According to ZUN, however, Mokou can also mean "Paint me red" and her name has no connection to Flandre Scarlet. In the right context, 'Kou' can also be translated as 'female phoenix bird', which is fitting, while 'Mo' can be translated as 'mourning' Her name would mean something similar to 'Mourning female Phoenix bird', a name somewhat fitting for her exiled life and events of her past.
Considering Mokou's surname and her back story as a daughter of an aristocrat charmed by the Kaguya-hime of legend, she is supposed to be the daughter of Kuramochi no Miko (車持皇子), who was given the quest for the Jeweled Branch of Hourai by Kaguya. His model is speculated to be Fujiwara no Fuhito, the factual founder of the Fujiwara clan, a powerful clan of regents during the 9th to 13th centuries.
Another note is at that time it was common for aristocrats to have "の" (pronounced no) between their surname and first name or their title and name. Thus the English equivalent of her name would be "Mokou of the Fujiwara Clan".
Mokou appears on the cover of Imperishable Night, silhouetted against the moon beneath the silhouette of Kaguya.
Mokou's character title is a pun on the title of ZUN's music CD, 蓬莱人形 ～ Dolls in Pseudo Paradise. She also uses a spellcard named Hourai Doll. Doesn't a certain someone else have a Hourai Doll too?
The paper talismans on Mokou's pants and hair ribbons might be wards to protect them against being burned by fire. Her shirt, which lacks those talismans, appears to have been burnt repeatedly, while the ribbons and pants show no such wear.
Because Mokou can't die, there are some parts of Gensokyo she cannot visit, such as Hakugyokurou. This is because being in the netherworld is equivalent to being dead, as quoted by Yuyuko in Perfect Cherry Blossom, and by Mokou in her confrontation with Yuyuko.
The fact it took the combined efforts of her, Kaguya, and several rabbits to put out a forest fire in her Bohemian Archive article implies that while she's capable of starting and increasing fire, she might not necessarily be able to extinguish it.
In Cage in Lunatic Runagate, she is shown as having had short, dark hair when she was mortal. No explanation of this is given.
Chapter 11 of Inaba of the Moon and Inaba of the Earth contains the first canonical depiction of a confrontation between Kaguya and Mokou. In it, they seem to have a danmaku fight with food.
She seems to have a good relationship with the members of Eientei despite her feud with Kaguya and is willing to lead the rabbits home. She also seems to have a good relationship with Keine Kamishirasawa.
In Canon, the Spellcard 199 in Imperishable Night might refer to Iwakasa, as she implied she felt regret over causing his death.
In the same Chapter in Cage in Lunatic Runagate: Chapter 4, it is shown that her immortality was derived indirectly from Iwakasa as well, as he saved her life prior to her immortality.
Mokou was planned to be a playable character in Touhou Hisoutensoku, but was scrapped due to a lack of time to balance her move set; ZUN stated in an interview she would have been overpowered and would unbalance the game (the same problem was encountered with plans to add Kaguya to the playable roster as well).
Mokou is one of the few characters wearing pants.
In contrast to Kaguya's terrible cooking, Mokou is actually good at what she does.
What! A trial of guts!? Oh, since when have humans been this foolish? You come all the way out here in the dead of night, with this lunatic full moon, and tons of youkai out, for a trial of guts, of all things!? It's a miracle that you weren't killed by youkai yet. A trial of guts, seriously! If you die, it'll be your guts the youkai will feast on---
~ Fujiwara no Mokou during Reimu and Yukari's Extra Scenario, when Reimu and Yukari go on a trial of guts
Rrrgh! I figured your trial of guts was suspicious. Of course she'd be behind this. Yeah, she goaded the two of you into getting rid of me this time, didn't she? She always, ALWAYS tries to do away with me! Argh, and wasn't she the one who gave me this inconvenient body in the first place!?
~ Fujiwara no Mokou during Reimu and Yukari's Extra Scenario, when Reimu mentioned Kaguya
Don't go thinking I'm some ordinary human. I won't die. I'm completely unable to die. Thanks to that lousy Kaguya. Yes, no matter how hard she tries to eliminate me, it won't do her any good at all. She knows that, and she still sends you messengers. This cannot get any more annoying.
~ Fujiwara no Mokou, complaining about Kaguya
A human who has drunk the Hourai Elixir will become unable to age or die. If another human eats that immortal human's guts, that person too will never age and never die. And if a ghost eats them...? An undying ghost will be born. Thus, the cycle of Hourai ends. They'll never achieve nirvana or reincarnate, and the elixir will stop along with them. Well, I guess it'd be fine if you didn't eat them.
~ Yuyuko Saigyouji, when asked by Youmu why a human who has drunk the Hourai Elixir is frightening
Actually... I can't die. To be devoid of death is to be devoid of life. Without the unnecessary states of life and death, I'm about as close as you can get to a pure human. So I may be like a phantom, in a certain sense.
~ Fujiwara no Mokou, explaining her abilities
That medicine? You mean the Hourai Elixir? That old thing. I used it all up a long, long time ago. Certainly, I stole that medicine, and am immortal even now. Kaguya still tries to kill me, but it's impossible to do that.
~ Fujiwara no Mokou, when asked about the Hourai Elixir
Why would you always wonder something like that? Immortality is like casting away the physical body. Your soul alone is what you actually are, and it's able to produce a new body. Souls don't have a particular size, so they can create a new body wherever they like. Conversely, a body without a soul would perish rather quickly, right?
~ Alice Margatroid, commenting on Mokou's abilities to her