In case you didn't understand

This is about Giju's life flashing before him. I'll explain plainly for ya! Yuju did die and Giju took his life at that cliff. The reason we see the woman is because it's his "mother" (his inner voice) telling him what to do basically. And it's why a lot of the manga is done so artistically, to show his memory of it all especially as a kid where memory can look weird. Now after opening the box Youngdo may have gone after the Mantis for Yuju himself or somehow ran into the mantis and was killed by him as mentioned in the beginning. Thats why we see him at the end with Giju and in that one scene with Giju as a kid. The only ones who survived this ordeal are Youngdo's friends and Elvis, which is why they seek out Mantis and Elvis is holding flowers by an ocean (I assume it's the cliff).

Essentially this is his interpretation of death and life flashing before your eyes and that he may see his mother, lover, and sister in death. This is such a beautiful and sad story ;;