An often seen and referenced character, Yukari is the youkai of boundaries. She was first introduced in Perfect Cherry Blossom as the sleepy mistress of Ran. During Imperishable Night Yukari teams up with Reimu to find those responsible for the fake moon. In Immaterial and Missing Power she actually sings her way from battle to battle stealing everyone's sake, wine, and brandy. Even right from the shrine's altar... however, this is all part of her strategy to save the day. Somehow.
Over a thousand years ago, Yukari organized an army of youkai and led them in an invasion of the Moon. The youkai were subsequently routed by the Lunarians, who possessed superior technology and magic, and forced to retreat to Earth. Since that time, no youkai have attempted to expand their territory beyond Gensokyo. While some speculate that Yukari organized the invasion out of selfish hunger for territory and power, others believe she did it to teach the youkai a lesson about starting wars of conquest; namely, that it was a bad idea.
About a thousand years ago, Yukari met and became friends with a human girl named Yuyuko. After Yuyuko died, Yukari continued to be friends with Yuyuko's ghost, although Yuyuko gradually forgot who she had been and why she had died. When Yuyuko requested that Yukari use her power to weaken the border between the Netherworld and Gensokyo, Yukari obliged. She knew Yuyuko's plan to make the Saigyou Ayakashi bloom could never succeed, but was too busy sleeping to say or do anything about it, or to put the border back to normal after everything was finished, which put her on a collision course with the heroines.
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 outside world (Reimu and Rinnosuke). She's possibly 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 Hinanai 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.