Theory: jhin has crush on Katarina

Jhin tells Katarina "your finale shall be a duet"

Jhin only uses the word "finale" other times to describe his finale and never uses it towards other people. He refers to others as masterpieces, art, works, etc.

It's also worth mentioning that jhins Q and Katarinas Q are close to the exact same thing. This adds to the "duet" because it can be in sync with each other.

Also when you think about jhin and Katarina working side by side in an actual game, it's pretty dam beautiful the kind of plays they can pull off with synergy.

Still not convinced? Jhin is a straight up asshole to everyone in his interactions besides Katarina.