Rin's ending is best acquired in a NG+ as she's fairly low on the affection priority.
Affection priority is Touko > Shizuku > Rin > Shion.
In NG+ you will get an option to see who gets affection in your dialogue choices, simply choose Rin all the time and avoid the other three girls and you will get locked into Rin's route. You have to keep increasing her affection during her route to get the true ending too.