The film's point of origin, missing from the published version
Played by Yoon Jin-seo. Lee Woo-jin's older sister; the same age as Oh Dae-su, at the same school (Sangnok High). Her screen time is brief — the rooftop flashback, the Hapcheon Dam jump. Yet the entire film is a twenty-four-year aftermath of her death. The very existence of the character Mido is the substitute Lee Woo-jin designs for her, and Oh Dae-su's fifteen years of imprisonment are a temporal equivalent of the roughly twenty-four years of Soo-ah's absence.
Her last line becomes the question-form of the film
Lee Soo-ah's last words — 'Woo-jin, I … you have to remember me. Promise? I have no regrets. Do you?' — set the question-form of the entire film. Every character in the film lives the next twenty-four years to answer that question. If Park Chan-wook's almost-never-showing-her-face handling of Soo-ah is intentional, then the mise-en-scène is that not-photographing-a-living-person is the strongest way to photograph a living person.
The rooftop bench, Sylvia Plath
In raw, the book Lee Soo-ah is reading on the schoolyard bench in the flashback is a poetry collection by the American poet Sylvia Plath. Plath suffered depression all her life and ended it at thirty by gassing herself in her own kitchen after four suicide attempts. Lee Soo-ah's favorite book being that collection reads as a soft piece of foreshadowing — she was overlaying herself onto a poet who, unable to attain an absolute love, chose death.