Sister Mary Louise is a nun who teaches Literature at Bishop O'Connel High School. She's Aislinn's favorite teacher.

Through the course of Wicked Lovely, Sister Mary Louise's classroom is featured twice: once in the context of discussing Othello; and once to discuss two poems related to faeries: "La Belle Dame Sans Merci" aka the "Beautiful Woman Without Pity" and "Goblin Market." Although the first classroom discussion isn't particluarly noteworthy or necessarily relevant to the plot; the second certainly is.

Although the knight in "La Belle Dame Sans Merci" doesn't realize he's dealing with a faery until it's too late; the sisters mentioned in "Goblin Market" do. Still one sister doesn't resist and suffers serious consequences the other sister tries to save her from.

Both poems share similarities between events ongoing in Wicked Lovely.
There are similarities between the "beautiful woman" and Keenan -- just as there are similarities between the fate of the Knight and Aislinn's possible fate. Likewise, "Goblin Market," with its emphasis on the temptation posed by the fey, even when a mortal is aware these are supernatural beings, speaks to the overall plot of Wicked Lovely.