OK, you can not lower a person's aggression with you any more than what they start off with. You can make them go higher, then bring them down, but never anything below what they start off with. So Kaori will always be 22% (or whatever) Neutral to Mac and there's nothing you can do to drop it a few points. You can push Kaori to like 89% and then if you leave her alone then her aggression will slowly drop back to 22% (or whatever she started off with), no more. If you make someone mad, then don't knock them down or bump into them in future races and they will lose 1-5 aggression with you with each race you have with them.
You can not save aggression values after you beat World Circuit. For example, if I Master Zoe and don't get anyone mad at me, then when I go play some World Circuit events and push JP alot, the aggression won't save with Zoe or anyone else you mastered. If you really want to make someone mad at you for more fun and you already mastered that person, you would need to start all over again with Tricky. I did that to get some enemies for Zoe, Eddie, and Seeiah cause it's much more fun with enemies!
Single Event aggresions won't save no matter what so you can push someone to death and they won't be any more mad at you in the next race. I'm not sure if they save or not if you haven't done WC, I doubt it though, but better safe then sorry.
Off-topic: My 199th post! I'm almost a Master!