peachy-arsenic

No, they just showed that she was dying cause of her piwers but Dong Baek helped her by erasing her memories (I think only the bad ones), but nothing past that. He also blackmailed the NIS agent into wiping her from the system. That’s the end of her story.

Like Aditi said, Dong Baek's sister is the Eraser. The girl that the Chairwoman's son and his friends raped was the sister's friend. At her funeral the sister read Chun Ki Soo's memories and found out the truth so she set out to kill everyone involved.
Chairwoman found out that sister was friends with the girl so she ordered her henchman to kill everyone involved, young or old. Henchman went to the house and saw DongBaek listening to the testimony that the dead girl had sent to his sister and he tried to kill him. Then his mom walked in so murderer ended up killing her instead.
DB ran away to his sister and saw the Shinbae warehouse murderers that she caused and ends up erasing his own memories out of trauma.

In the end he kinda heals his brain issues by accepting his repressed memories and finds evidence of the girl's rape in his sister's memories. Chairwoman and son are jailed, the son commits suicide.

The murderer wasn't going to kill DongBake or his mom, he only wanted to kill the sister because he thought that she might know something about the rape. He was going to leave but then Dong Baek found the testimony that the girl had sent so murderer was about to kill him and then the mom walked in. The reason she was killed was because she walked in on the chairwoman's henchman tryna commit murder and also maybe because she also heard the testimony.

It was purely bad timing.