[list][*][u]Name[/u]: Fujiwara no Mokou
[*][u]Appearance[/u]: [url=https://i.imgur.com/f2F3nAe.png]Figure of the Person of Hourai[/url]. 
[*][u]Ability/Skills[/u]: The very most notable ability that Mokou has in her possession is simply not dying, nor ageing--due to drinking the Hourai Elixir, the concept of death has been removed from her being and no matter what happens to her, she'll be fine pretty shortly. This does not seem to render her completely immune to the consequences of her actions; when she originally drank the elixir her hair was short and still black, and [i]obviously[/i] dying still hurts, which leads to the most obvious path to defeat. This also appears to render her immune to slow, drawn-out death by neglect--starving is painful, but not actually fatal. She can also do housework reasonably well (as you'd expect), but somehow isn't the best at identifying what might be a deadly poison. 
Aside from the useful ability to stay alive and the ubiquitous flight, Mokou primarily focuses on fire, honed through over a thousand years of life and presumably an interest in phoenix motifs. [i]Lots[/i] of fire, which she isn't fully able to extinguish if it catches other things alight. Whilst her hair and hakama appear to be protected against this fate, she doesn't appear to have taken the time to extend the same courtesy to her shirts.
Although her abilities are conceptually quite simple, and her durability about on par with a common fairy's, Mokou is considerably dangerous simply through the ability to disregard her own safety and keep attacking.
[*][u]History[/u]: As her name suggests, and her peripheral involvement in the Tale of the Bamboo Cutter implies, Mokou can only be a daughter of the Heian nobility: almost certainly the unnamed and untraced fifth daughter of Fujiwara no Fuhito, himself in one theory a child of Emperor Tenji, and the Prince Kuramochi of the story, who tried to fake the Jewelled Branch and was discovered. Although hardly a public figure, Mokou could hardly let this insult to her family go... or, at least, that's the retroactive claim.
In truth, although Mokou wound up following the soldiers assigned with burning the Hourai Elixir, a young girl could scarcely climb a mountain on her own, without equipment, and accompanied them up to the summit. When most of the soldiers fought each other, or were killed by Sakuyahime, only Mokou and Iwakasa were left to take the elixir to another mountain. Of course, this is when the desire for immortality became quite acute and Mokou murdered Iwakasa for the medicine.
As an immortal in a human body, there wasn't really a place for Mokou in society any longer, and she essentially dropped out of sight of the world. Then there was a period of centuries where she would kill anyone that got in her way and several more where she did nothing whatsoever. The insult towards her father lay forgotten until she arrived to Gensokyo and found the person behind it once more: cue [i]another[/i] three hundred years of fighting one another under the pretext of revenge, but more because there's simply nothing to [i]do[/i].
With Eientei no longer being so static and unchanging, Mokou too seems to have started to reach out to other people. A little. Mostly, this has her serving as a guide out of the forest, but she's started to get caught up in other incidents.
[*][u]Personality[/u]: Mokou is something of a loner, to put it mildly. Most of the people that she guides out of the bamboo forest she won't even say a word to, though she'll listen to them fine. With people that she has some reason to spend any time with--or fight, which appear to be more or less synonymous--Mokou is a lot more exuberant, if definitely rough around the edges. Though she was both extremely apathetic in the past and extremely violent, Mokou is [i]something[/i] of a nicer person nowadays, although she does have a tendency to get oddly curious about anything that might actually be able to kill her.
That isn't to say she's not still cynical, though.
[*][u]Other[/u]:[/list]