Magami Tokiko is Kamui's aunt, and Toru's sister. She also bears a striking resemblance to her sister. She first appears in X/1999 #1, 'Prelude'.
As far as I recall, she wasn't in it.
Her presence here seems to be one for Kamui's reassurance. She's the nurse at the school hospital, and she's first seen when Kotori wakes up after she collapsed coming from the roof of the building. She knows of Kamui, although when Kotori questions her about who brought her in, Tokiko feigns ignorance. Later, at Fuma and Kotori's father's funeral, she appears, introduces herself and takes Sorata and Kamui back to her house. Here she explains about how come Kamui didn't know he had an aunt, and tells him more about his mother. They leave her, when they feel a kekkai open, and when they go back to find her the house is burned to the ground. Kasumi Karen tells them that Tokiko told her to burn the house as she was being taken away by some men, and used a spirit tongue to tell her. She's not seen again until book 6, 'Duet', when she collapses, bloodied up, outide Fuma's house. She explains to Kamui and Fuma how she was destined to die like this - She gives birth to the second Holy Sword, drawing it out of her own stomach, then dies.
X Japan song: