I copy this on Wikipedia:
"Ikiryō is a disembodied spirit or ghost in Japanese popular belief and fiction that leaves the body of a living person and subsequently haunts other people or places, sometimes across great distances. according to mythology, the ikiryō does not necessarily act out of spite or vengefulness, and stories are told of the ikiryō who bears no grudge or poses no real threat. In recorded examples, the spirit sometimes takes possession of another person's body for motives other than vengeance, such as love and infatuation. A person's ikiryō may also leave the body (often very shortly before death) to manifest its presence around loved ones, friends, and/or acquaintances."
So it's like an astral projection of Mayumi's grudge. Meeting her old bully again, even if she is happy now, her deep hatred towards her old bully is so strong, that it manifests to haunt her old bully and her child.