Relia's ending is pretty easy to get and works the same as everyone else's endings, in which you need to do her PAs and get affection points, but you can get hers along with any of the other endings above. If you have enough affection points after beating the final boss, you'll get Relia's ending after the first character ending you get, but if not, then you can use Love Potion No. 256 to raise her affection until you do.