Yukari Yakumo is a legendary youkai who is able to manipulate boundaries. She is the master of Ran Yakumo and Chen. She is called the gap youkai. She's also a yandere. 

She 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 is simply one of the most mysterious youkai living in Gensokyo. She lies about her age, saying she's 19 while she's actually older than Gensokyo itself.