Mother

Do-joon was arrested because circumstantial evidence pointed to him as the killer. He had followed Ah-jung to an abandoned building on the night of her death, and his intellectual disability made him an easy target for police pressure. However, the truth later emerges that Do-joon did not intentionally kill her, it was an accident triggered by her constant taunting.

Earlier scenes show Do-joon is sensitive to ridicule and prone to violent reactions when mocked.

Ah-jung died on the rooftop of an abandoned building after Do-joon followed her there. The junk collector later revealed that Do-joon accidentally killed her when she repeatedly taunted him about his intellectual disability, calling him names. In a moment of uncontrolled rage, Do-joon pushed her, causing her to fall and die. This was not premeditated murder but a tragic accident.

Do-joon's violent reactions to ridicule were established early in the film when he vandalized a car after a near-accident.

When Do-joon was a child, his mother attempted to poison both of them because she could no longer bear the hardships of raising a son with intellectual disability alone. During this traumatic event, she told Do-joon she never wanted to see him again. This dark secret resurfaces when Do-joon recalls it during a prison visit, revealing the troubled history between them.

The mother's extreme dedication to protecting Do-joon hints at underlying guilt and a need to make up for her past attempt.

When the mother broke into Jin-tae's home searching for evidence, she found what she believed was a blood-stained golf club that she thought was the murder weapon. However, it turned out to be only lipstick, not blood. Jin-tae used this discovery to blackmail the mother into paying him for his investigative help.

This false lead demonstrates the mother's desperation and willingness to believe anyone could be the culprit except her son.

Jong-pal was a boy who had escaped from a sanatorium and had been involved with Ah-jung romantically. The police arrested him for Ah-jung's murder, claiming he was the true killer. However, Jong-pal insisted the blood on his shirt was from a consensual encounter with Ah-jung, not from killing her. He became a scapegoat for the police to close the case.

Earlier investigation revealed Ah-jung had a reputation for being promiscuous and was involved with Jong-pal.

At the end, the mother uses her own acupuncture technique to erase her memories because she cannot bear the guilt of knowing the truth about her son's accidental involvement in Ah-jung's death. She also carries the trauma of her past attempt to poison him. By erasing these memories, she chooses to live in ignorant bliss rather than face the painful reality they share.

The acupuncture treatments throughout the film were established as a way to help people forget painful memories.

The junk collector was an elderly man who collected scrap near the abandoned building. He witnessed the entire incident: Do-joon following Ah-jung to the rooftop, her constant taunting, and Do-joon's violent push that caused her to fall to her death. He only revealed this truth when the mother pretended to offer him medical care to extract information.

Do-joon identifies the collector from photos on Ah-jung's phone, setting up this crucial revelation.

Do-joon followed Ah-jung after encountering her on his way home from a night out. While the exact reason is ambiguous, Ah-jung was known for her promiscuous reputation, and Do-joon, despite his intellectual disability, may have been drawn to her. His following her led to the tragic sequence of events that unfold.

Earlier scenes establish Do-joon's vulnerability to peer influence and his mother's concerns about his associations.

The mother provided unlicensed acupuncture treatments to women in the community, helping them forget painful memories. This ability to erase memories becomes central to the ending when she uses it on herself. The treatments represent both her desire to help others escape pain and her ultimate choice to escape her own unbearable truth.

The repeated shots of the mother performing acupuncture establish this as a key thematic element throughout the film.

The mother joins other parents in a celebratory dance on the bus during the 'Thank-You Parents' tour as a manifestation of her choice to erase her memories and live without the burden of truth. This dancing represents her acceptance of ignorance and her desperate attempt to find happiness, even if it is built on a foundation of denial and shared guilt.

The contrast between this joyful celebration and the dark truths revealed throughout creates the film's bittersweet, haunting ending.