Yukari Yakumo is the youkai of boundaries. She was first introduced in Perfect Cherry Blossom as the sleepy mistress of Ran Yakumo. 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.
In the Ultimate Universe stories she causes trouble for the heroes but she and Rosalina secretly want the heroes to explore new worlds for them