User blog comment:Crimson and Darkness/DxD Updates 2~/@comment-30924344-20170613174327/@comment-1359322-20170614091627

For Bova, Ishibumi planned to add him in before Ise became a High-class Devil but since Ise is aiming for a all female peerage, Ishibumi decided to make Tannin into Ise's follower instead. Another reason is because Ishibumi views a human looking male subordinate doesn't suit Ise, so went with a Dragon.

Ishibumi also planned for Ouryuu for a while already, he wanted another male kouhai (junior) that looks up to Ise. Ishibumi also said he originally planned for more back-setting but since he doesn't know how far he'll write DxD, it may be good for another story (i guess we may really be getting more spin-offs on DxD after -SD-).

Millarca is planned simultaneously as Ouryuu, apparently Ishibumi wanted to write on a vampire princess heroine, so he introduced Millarca. I guess this is why Millarca is with Ouryuu, Ishibumi probably wanted Millarca as a main heroine for her own story. I think we may actually get a spin-off focusing on Ouryuu.

Maybe Ishibumi is really planning on expanding the DxDverse with other stories focusing on other characters.