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.)
- Kaguya Houraisan (Bitter enemy)
- Keine Kamishirasawa (Friend)
- 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.