Michaeli Penhaligon was Arthur Penhaligon's older sister. She was still living with her parents and Arthur because her college was nearby.
In Grim Tuesday, her college was closed and the building was sold to pay the college's debts. She was informed by e-mail and was outraged by this. It turned out it was all Grim Tuesday's doing and she returned to her classes.
In Sir Tuesday, Michaeli apparently is the only one in the Penhaligon's house when the quarantine was issued, as only she noticed Leaf. She was shocked when Suzy Turquoise Blue materializated inside Arthur's room, but later agreed to help them and explain to Sylvia that Leaf was hurt and needed medical care.