In case you haven't noticed, the final dialogue with her, she reveals her true name to be Saori(Mandy). She is jealous twin sister of her. She died putting curse on her and became a ghost. She is the reason Kaori (Wendy) ended up in the asylum. Helping her at this moment leads to Kaori's death resulting in her appearing as ghost in old LSH.
You see her in original LSH tutorial level. After you capture Kaori, reveals herself and attacks Taichi(Ted).
Kasumi is distant relative of Taichi. She used to be daughter of head of village Silver Spring village.