Yukari Yakumo
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 (such as Suika and Yuyuko) 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; 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.