This is a story from over 500 years ago. If I were to name the one youkai that is the most youkai-like of youkai, that would be Yukari Yakumo. After all, she's a typical one; although she claims that she is to be hibernating through the winter, we have only her word on it and it's unknown where she lives, so this is unconfirmed. She's said to now look the same as she had appeared back then. Her appearance is no different than that of a human's appearance. Yukari's human friendship level is average. Aside from the fundamental dangerousness of her ability, she also tends to appear and disappear without a warning, her personality seems to lack common sense, and her behavioral principles are completely different from those of humans, making her a youkai not to be dealt with. Her main time of activity is night, and she sleeps in the daytime. She is often seen with a large parasol and prefers extravagant clothes.
If there were no boundaries, everything would probably exist as a single enormous object. Were it not for the Great Barrier, even Gensokyo itself wouldn't exist. If there was no water surface, there could be no lake. If there was no sky line, neither mountain nor sky could exist. She is also capable of going anywhere through a gap in space, whether it be her entire body or only a part of her. This barrier literally is the boundary between inside and outside. In addition, she is possessed of superhuman intellect, and is especially good at mathematics.
Yukari Yakumo is actually quite talkative person, eager to teach you about herself or about Gensokyo. However, most of her stories are impossible to prove or confirm, so no one can say if what she says is true. She alos said the her house is located at the boundary between Gensokyo and the outside world, just like the Hakurei Shrine, but no one has ever actually seen it. Nobody can follow her through the gaps, so it's not known if it's true or not. This one, of course, refers to the Great Hakurei Barrier. A rumor has it that she is a youkai from outside, 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 outside world is in fact Yukari's dream. Others say that her house is located in the outside world. Well, Yukari's a youkai, it's no wonder she's superhuman. The humans probably have no possible chance of winning against her.