Even if they have opposite personalities somehow they compliment each other. Look at the act of teleporting thingy and when he said "world's sweetest" they did the same thing. Actors do have the "the look test" to see their compatibility on screen. They are 100% in every aspect compatible, opposites and complimentary. I don't know why it is just magic. Can't explain it.
They are so freaking adorable together! Such a brilliant and explosive chemistry, haven’t seen like it before, so comfortable with each other. I remember them working together in School detective. Hyeri is very good in complementing others, that’s why most people she encountered tends to like her. Just a pleasant and down to earth attitude.