Sister Joan is a Carmelite nun who acts as chaperon for Moira Parker when she comes to the hospital in the Season 8 episode Chase. She was portrayed by actress Valeri Ross.
Robert Chase sends Sister Joan out of the examination room, ostensibly so Moira can have some privacy while he performs a breast exam. However, when she leaves, Chase reveals his real motive was to determine if Moira was suffering from extreme anxiety because of her upcoming decision to take her vows.
Later, Sister Joan stays with Moira while she is hospitalized, and comes to pick her up when she is released the first time.
Moira decides she wants to leave the convent, but winds up being re-admitted when she suffers a carotid artery dissection. During surgery to correct it, she has what she interprets as a religious experience and decides to return to monastic life despite Chase's urging. Sister Joan comes to pick her up again when she's released.