しらせ

Highly elusive, Yukito meets her by chance in the ruined greenhouse. Her name is not on the student register and no current student has heard of her, leading Yukito to suspect if she's really a ghost. 

She is referred to as the "Witch's spirit" (魔女の幽霊) and is rumoured to turn female students who meet her into witches.

 Full name Maya Shirase, the younger twin sister of the school director Maya Kisashi. Born with a weak constitution, she was granted her wish to live by the long-standing witch phenomenon. Animals and humans in close proximity to her would have their lifeforce drained and transferred to Shirase for sustenance; the end-result of this power would have granted Shirase eternal youth and immortality. The school director at the time recognised Shirase's threat to the student body and made the decision to dispose of her, seemingly leading to her death by immolation from devout teachers and students. /