All the Signs Yi-Jin Is Min-Chae's Father on "Twenty-Five Twenty-One"

In episode 12, when everyone gathers at Yi-Jin's apartment to say their goodbyes before the world possibly ends because of the Y2K bug, Yi-Jin's white cassette player can be seen behind Ji-Woong. Fans noticed that the cassette player can be seen among Hee-Do's belongings in the present day. They know it's not Hee-Do's because Hee-Do had a black cassette player. This discovery, along with the photos Hee-Do has and the decorations in Min-Chae's room, further suggest that Yi-Jin and Hee-Do are together in present day and Yi-Jin is Min-Chae's father.