If I were to name one youkai that is the most youkai-like of youkai, that would be Yukari Yakumo.

She is a youkai with whom few would choose to associate. Besides the fundamental danger of her ability, she tends to be elusive and her behavioral principles are completely different from those of humans, to name a few of her issues.

Her appearance is not especially different from that of a human.

She prefers extravagant clothes, and is often seen with a large parasol.

Her main time of activity being at night, she sleeps in the daytime.

All in all, she is a traditional youkai (*1).

Though she claims to hibernate through the winter, we have only her word on it and it's unknown where she lives, so this is unconfirmed.

There is an article in the Youkai Encyclopedia from Aichi's (*2) Gensokyo Chronicle from long ago that seems to describe her.

It appears she existed back then as well.

1: Recently, as the number of night humans increases, so does the number of daytime youkai.

2: The Gensokyo Chronicles written by the first member of the Hieda line, more than 1200 years ago.

The ability to manipulate boundaries is a terrifying ability capable of fundamentally undermining reality.

As far as we know, everything is built upon the existence of boundaries.

If there was no water surface, there could be no lake.

If there was no sky line, neither mountain nor sky could exist.

Were it not for the Great Barrier, even Gensokyo itself wouldn't exist.

If there were no boundaries, everything would probably exist as a single enormous object.

Thus, the ability to manipulate boundaries is by logic an ability of creation and destruction.

It essentially creates a new being, or rejects the existence of a being.

Among the abilities youkai are known to possess, this is one of the most dangerous, being comparable even to the power of gods.

She is also capable of moving to an entirely separate location between gaps in space, whether it be her entire body or only a part of her.

It's said that this ability is not limited to physical space, but also applies to pictures, others' dreams, and even stories.

In addition, she possesses superhuman (*3) intellect, especially concerning mathematics.

Furthermore, due to her long lifespan, she has a wealth of knowledge and experience.

3: She's a youkai, so it's no wonder she is superhuman.

She is actually quite talkative, and is willing to teach you about herself or about Gensokyo.

However, most of her stories are impossible to prove or confirm, so no one can say how many of them are true.

Her house is said to be located at the boundary between Gensokyo and the outside world, just like the Hakurei Shrine, but no one has ever actually seen it.

This is because she moves through gaps in space, so she seems to appear out of nowhere and disappear into nothing.

No one can follow her through the gaps, so it's not known if it's true or not (*4).

In addition, she rarely does things herself, leaving practically all of her work to a beast she controls as a familiar instead.

The familiar acts in her place while she is sleeping, mostly during the day or in winter.

Also, as she has lived a long life, she has many friends that are similarly long-lived.

Most of her friends are youkai of the strongest class, like Yuyuko Saigyouji of the netherworld and the oni Suika Ibuki (both described later).

4: There are rumors her house is located in the outside world.

Gensokyo is separated from the outside by a large magical barrier.

This barrier could be seen as the boundary between inside and outside.

However, the inside world and the outside world are still physically contiguous, so it may be logically said that the barrier created the place that we call Gensokyo.

That also means that as long as the youkai of boundaries exists, she can create or destroy a place like Gensokyo whenever she desires.

It can be claimed that the fact that a youkai of boundaries exists in somewhere Gensokyo, a land surrounded by an enormous boundary, implies that she must have some deep, vital connection to its formation.

Strangely, this youkai rarely attacks humans.

A youkai that never attacks humans is by definition not a youkai, so it is assumed that she does somewhere.

As she can freely visit the outside world, it's speculated that she attacks humans in the outside world (*5).

5: There are other rumors that she is a youkai from the outside world, and she simply comes to here for play. Indeed, she may spend the winters and the daytime, when she claims to be sleeping, in the outside world, and some even say that the world in Yukari's dreams IS the outside world.

This is a story from over 1000 years ago.

It is said that she once invaded the moon by modifying the boundary between truth and falsehood and plunging into the moon's reflection on the lake's surface.

Although she went in with an army of impudent youkai, they were crushed by the Moon's advanced weaponry.

It is believed because of that incident, youkai usually do not choose to attack outside their territory.

The stories of this event are also what made her powers widely known among humans.

A story from over 500 years ago.

The project refers to a plan designed and implemented by Yukari in order to restore power to the youkai of Gensokyo, who felt pressured by the increase in the human population.

Until then Gensokyo was merely an isolated mountain town, but she planned to create the boundary of phantasm and substance around Gensokyo, thus logically creating a new world.

By establishing Gensokyo as an illusionary world, and the outside as the real world, the youkai weakened by the outside world would naturally flock to Gensokyo; this was a truly epochal event.

At this time, it seems that youkai from outside of Japan immigrated to Gensokyo, as well.

The effects of the boundary continue even now.

The cleverness of the plan lies in allowing youkai to disappear in the outside world while permitting them to thrive within Gensokyo.

As long as the outside world remains in the hands of the humans, Gensokyo will be ruled by the youkai.

This, of course, refers to the Great Hakurei Barrier (*6).

It is said that this youkai was one of those who suggested the creation of the Great Barrier and created what is now Gensokyo.

However, it appears that there was some opposition to this plan from a number of youkai, and there was open conflict between them.

As this conflict distracted youkai from their normal attacks against humans, the Great Barrier has received great praise from those living in Gensokyo.

Word of the usefulness of the Great Barrier has now circulated among the youkai, and there are scarcely any that oppose its existence now.

The Phantom Disturbance refers to a time when the border between the Netherworld and the world of the living weakened, making it easy for the respective denizens of each to cross through it.

It is obvious it was the doings of this youkai.

Since this was the act of an extremely wise youkai, there was likely some extraordinary benefit to youkai society in doing this. (*7)

As this is an ongoing incident, the matter remains unresolved.

6: The boundary separating Gensokyo and the outside world. It is said it is extremely difficult for youkai or humans to pass through, and so logically it creates a separate world.

7: I have been unable to confirm this in my lifetime.

"She makes my food disappear right out from under my nose, even when I'm careful about it. I wish she'd stop." (Reimu Hakurei)

When you're not looking she can slip her hand through a gap in space and take things from far away.

"She keeps poppin' into my house and I wish she'd cut it out. Can't she at least use the door like a normal person?" (Marisa Kirisame)

One should exercise restraint when using the power of teleportation.

She seldom appears in the human village.

Barring unknown circumstances, she likely doesn't attack normal humans very often.

No matter how confident one is in their abilities, it would be wise not to attempt to exterminate this youkai.

Though, she would likely make allowances for the power of her opponent so as not to allow the battle to become too one-sided.

In any case, there is neither any sort of defense against nor weak point in the ability to manipulate boundaries.

Her intellect far surpasses those of humans, and her physical strength is what you'd expect from a youkai.

Humans probably have no chance of winning against her.

Excepting, of course, the countermeasure of approaching her in a polite manner.