As much as I do appreciate how DxD isn't scared to kill off villains, a LOT of them should have gotten more development. Especially Raynare due to how important she is in the long run. Plus with her hinted inferiority complex, I think she could have played off Issei rather well.

Interesting.

If you want write her being good, I suggest doing it gradually. Make her earn their forgiveness, Issei moreso.

Don't let her fall in love too quick. Break her down, make her suffer a bit, give her long-term conflict, give her the same mental trauma Issei had for her.

Stuff like that.

Your story has potential. All you need to do is write it well and do some good storytelling.