Music and time travel are the main themes of both dramas. In Twinkling Watermelon duet father and son, who travelled from the future are decided to create a band. In Lovely Runner male lead is part of a music band. In Twinkling Watermelon son is trying to save his father from accident, which caused his father, loss of hearing. In Lovely Runner female lead is the one who is trying to save her favourite idol from death. Both dramas are plenty of humour, emotion and great music.

Ghosts theme is a main theme of two dramas. In both dramas one of a leading characters is able to see a ghosts. The seeing character in these dramas can protect themselves from ghosts by touching and being present with the other leading character.

Both dramas are happening in the same timeline. Beginning of both stories is the year 1997.
Main characters of both of the stories are a group of friends from the high school. Another similarity is that in both of the shows main focus is on the female main character's journey to pursue her passion. In Reply 1997 that's music and in Twenty Five Twenty One is fencing. Connection between main characters and following their life journey over the years is another of the resemblance in both shows.