Yukari is an elder yōkai who dwells on the border of Gensokyo, and may predate the existence of the realm itself. A good friend of Yuyuko's and an acquaintance of Reimu's. She is the self-appointed stewardess of the seal between Gensokyo and the outside world, but doesn't do a particularly good job of it...
Most of the time, she is asleep, leaving all the work to her shikigami Ran and Ran's shikigami Chen. When she isn't, she tends to amuse herself by pulling in unprepared humans from the outside world and stranding them in Gensokyo. A very irresponsible yōkai.
Her ability is the manipulation of boundaries, and such boundaries distinguish her spellcards. Often, though, she just summons her shikigami and various others to create her danmaku for her.
@Hakurei the Strike Witches club has a weekly Club DP contest. The rules are: you will submit 1 picture in this thread per week that you would like to see as Club Display Picture. I will select the first 4 pic submissions for that week and put them in a poll, then you will vote for the 1 picture that you want (the poll will be open for 6 days). If your picture is not chosen as the DP that is among the selections in the poll, and you want to use it again, you will have to resubmit that picture.
Other ideas: voting on a top Yukari related doujinshi, video, arrange etc.