Yukari Yakumo is one of the oldest and most powerful youkai in all of Gensokyo. She is in fact older than the history of Gensokyo and therefore very knowledgeable and experienced in all matters concerning it as well as the outside world. At the same time she seems to lack common sense from a human perspective. Her behavior is very unpredictable and she can either be a heroine or a villain depending on her impulses. She is also a very typical youkai in that she likes to sleep a lot and mostly just enjoy herself. She even has a shikigami (a summoned spirit) called Ran to take care of the daily activities around the house during her long naps.
Her house is located somewhere on the border of Gensokyo, but no one knows where, because Yukari's power to control boundaries allows her to travel anywhere through a gap in space. It's a power comparable to those of the gods, with it's ability to fundamentally alter reality. It's not limited to physical space either, it's said to be able to alter dreams as well. Because of her tremendous power she often prefers to toy with her opponents rather than fight with her full strength.
Yukari has appeared as a playable character and a boss in several Touhou games.