Mystery, other-worldly, reincarnation, love, friendship, BL, bromance, separate entities living in the same world, loyalty

Ghosts, supernatural, love, mystery. Similar themes of death and after-life. Some similar themes of romance

Themes of the supernatural, other worldly love, ghost and spirits and friendships/ loyalty. Similar tropes and themes.

Both dramas deal with death and family. Both have themes related to relationships and deepening mystery.

9/1 is a continuation from 9
Characters from 9 are now adults with children
Exactly the same themes of family, love, conflict

Coming of age, high school students, finding oneself, basically the same theme of teen and discovery.

Coming of age.
Forging friendships
School/ life balance
Reality

Similar themes of love and relationship problems mixed with a bit of college life

Both are about relationships, college life and society's view on certain types of relationships.

University, love, relationships. Same themes, different execution. School-life-relationship balance.

Similar threads of past lives, honour, after-life, ancestry and culture

Similar plots about developing love and relationships. While Grey Rainbow is a little more, well, grey, SOTUS is more upbeat and positive. If you liked the other you will like this one!