Border of Phantasm
Species: Youkai
Ability: Manipulation of boundaries
Age: At least 1,200 years old, older than the history of Gensokyo itself
Occupation: Unknown; she sleeps most of the time
Location: Somewhere along the boundaries of Gensokyo
Yukari Yakumo is one of the oldest youkai in Gensokyo, one of the strongest of youkai, and is praised as one of the wisest of youkai. She was one who proposed the Great Hakurei Barrier, which covers Gensokyo, and is therefore tied to Gensokyo's creation. She also performs surveillance and safety administration (mainly for Ran Yakumo).
She first showed up in the phantasm stage of Perfect Cherry Blossom with Ran Yakumo serving her as shikigami. She is called the "sukima youkai", although "sukima youkai" is not the name of a species, as she is more of a one-of-a-kind type of youkai. She was also a playable character in Imperishable Night as well as the fighting games, an Extra Level target in Shoot the Bullet, as well as a support character for Reimu Hakurei in Subterranean Animism, thus continuing to make occasional appearances.
Yukari is known for being a very youkai-like youkai who sleeps all day and lives for the enjoyment of life. Though she rarely leaves her house, she's well-connected and is acquainted with most of the most powerful youkai as well as anyone having anything to do with the Hakurei Border or the outside world. She's possibly one of the most powerful youkai in Gensokyo and has a tendency to toy with her opponents rather than use her full power from the start. If anyone is suited for the role of "mastermind", it's Yukari, and she often has a great deal of insight and understanding about whatever happens in Gensokyo, and possesses considerable intellectual prowess, as well. Though she seems flaky and unreliable most of the time, when the safety or security of Gensokyo is at stake, she won't hesitate to get involved, even if it means using others to do the work for her. For example, in Scarlet Weather Rhapsody, she treats Tenshi Hinanawi with much hostility after finding out the plot of the Heavens. She could easily be either a villain or a heroine, depending on her whims.
According to Perfect Memento in Strict Sense, she seems to rarely attack humans. She excels at mathematics and is experienced due to her long life.
Yukari deals with problems by manipulating others to act to resolve things for her. She seems to rarely act directly except in particularly egregious situations. In all cases, she rarely lets anyone know what her true aims or goals are, and sometimes these goals are extremely abstract. Yukari has a well-deserved reputation of being fickle, whimsical, and lying often, and thus many characters in Gensokyo dislike her. For whatever reason, Yukari takes it all in stride and does not seriously deny any of those comments. She seems to actively try to keep her more obviously-altruistic actions a secret, such as her donations to the Hakurei Shrine in Strange and Bright Nature Deity. She possibly cultivates this image to ensure that people continue to fear her as a youkai, as this is what her true goal for a particularly complex gambit is revealed to be at the end of Cage in Lunatic Runagate.
As far as her knowledge of technology, in Chapter 11 and Chapter 12 of Curiosities of Lotus Asia, when Rinnosuke Morichika had an iPod in his shop, only Yukari knew how to actually use it, and further knew that it was considered a trendy item in the outer world. She similarly knew about the Nintendo Game Boy and DS.
Because of her understanding of technology from the outside world, there are rumors that she lives in the outside world from time to time. It is confirmed that she at least visits there. Reimu mentions in Curiosities of Lotus Asia Chapter 24 that Yukari told her that she gets to enjoy twice the amount of cherry blossom viewings a year because she also goes to watch the blossoms in the outside world, which occurs at a different time from Gensokyo's. In addition, Yukari at the end of Chapter 25 of Strange and Bright Nature Deity is shown viewing the Hakurei Shrine from the outside world (some panels focus specifically on pointing out there are cars driving by).
Marisa Kirisame once stated that Yukari uses a small box to talk to her shikigami when they are far away from her, which led Rinnosuke to believe that she owns a cell phone. She has given characters cell-phone-like devices in Subterranean Animism. Yukari mentions in Oriental Sacred Place chapter 10 that radio towers fall into Gensokyo every once in a while. Presumably her cell phones use those in order to work.