how come my relationship status with yuna wont go above 20
When you start the night, go to "living room" then "give gift" and finally "chat" - this is how you can get a hint from her what to do next
Same thing when you hit rel 40
If you want ready solution scroll down:
Rel 20 - cook for her
Rel 40 - buy running shoes (go running at night in the city, then buy them from Syn the vendor)