A person every choice of whose was someone else's design
The chat friend Evergreen with whom she talked for five years, the anonymous benefactor who sent help every time she — raised without parents — had a hard time, and at some point the person who, without ever showing his face, told her he loved her — were all the same face of the man who used her. Mido does not know this to the end of the film, or pretends not to. That is the double cruelty Park Chan-wook designed.
Real name = Yeon-hee, Woo-jin since age four
Mido's real name is Oh Yeon-hee. The four-year-old at her birthday at the opening of the film, 1988. Woo-jin has been beside her, as her anonymous benefactor, since she was four. In the film the name 'Yeon-hee' is spoken once across the father's payphone and never called again. Screenwriter Jeong Seo-kyeong's single line — 'is Mido really unaware?' — flips this character at one stroke from a plane into a solid.
Park Chan-wook's own self-assessment — that he felt sorry for Mido
In the twentieth-anniversary Cine21 conversation Park said Kang Hye-jung's superb performance made Mido feel like a stronger character than the script supplied, and that he felt sorry for Mido's situation — excluded from the film's central secret. His own framing is that JSA had been male-centered, so he wanted his next work to be wound up with a female protagonist — which became Sympathy for Lady Vengeance.