Jade Regent by Xzaral
The Relationship rules themselves as presented in the Adventure Path are optional. It is up to the group on whether they will be used or not. They do provide a mechanical bonus to the game. Also the Romance rules are an additional optional rule to be used. This will be used based on the comfort level of the group.
The only addendum I see that I would add if the rules are used is that gifts given to increase relationship is capped at 1 per level, but does not have to be given at that level. Now I’m not going to allow someone to wait until level 10 and suddenly want to do 10 rolls all at once (subsequent rolls would probably get penalties), but will allow incidents where you can’t RP a proper scenario during the course of a level so you don’t lose the bonus.