From The PSPedia2
Each time you clear a mission with a partner character or MySynth that you invited to the party, you will earn points which go toward an invisible stat known as a partnership rating. Points earned for this rating are based on the grade you receive after clearing a mission; mission difficulty has no effect on it. Please reference the table below to see how many points are earned for each grade.
| Mission grade || Points earned
| C || 0
| B || +1
| A || +2
| S || +3
As your rating with an NPC increases and reaches (or surpasses) specific milestones, you may receive an item directly from that character after completing a mission with a grade of S. The charts below detail the particular items each character may present you based on your current rating with them. Keep in mind that items from lower ratings are not lost as you progress, but instead are pooled together to give you numerous potential rewards. As an example, even if you have surpassed an NPC's highest rating of 100 points, you still may receive an item from any of their lower tiers.