It took me a really long time to get Kadin's good ending, way back when he was first released. I don't know if these answers have changed at all since then, but these are the choices I had to make to get it. I hope I won't get in trouble for posting this here?? but I remember how annoyed I was when I couldn't get his good ending, so... I hope it helps :)
Kadin - Gum Attack
Let him.
Tomorrow after lunch.
Kadin - Fluoro Pink
Continue waiting for him.
Okay
Kadin - Prank Buddies
Kadin - Bike Revenge
Paper
Kadin - Pink Spray
Yes
Cut the skewers.
Kadin - Masters of Glue
Help Dad with the door.
Have a visit at the restaurant where I work.
Kadin - Dinner Date Mission
Don't go.
Big chance.
Kadin - Unique Cupcakes
Play board games.
Catch up to him.
Kadin - Unexpected Customers
Yes, tell him now.
GOOD ENDING