Look, I'll level with you; I'm taking liberty with the canon of DxD and getting rid of a few characters that either:
1) Contribute nothing to the narrative.
Or
2) The story is made worse by their existence.
Or
3) Just so I can have fanservice.
Three examples are Ophis, Zeoticus, and Millicas.
Zeoticus and Millicas are for reasons "1" and "3". Self explanatory, so I will not need to elaborate.
Ophis is for reason "2". She was boring, inconsequential, and didn't make any sense. The khaos Brigade members already have a common interest, so she isn't even needed to unite them. She didn't even start the khaos brigade, it was Satanael. Her goal is also confusing: the dimensional gap is infinite, so why is Great red a problem? She seems Okay with the Wizards Of Oz and the endless rating games which been happening for almost 500 years. Even then, how is terrorising a random world gonna achieve her goal?
So, yeah, she's out.
I'm also going to forgo certain plot points that I can't incorporate the MC in, like Volume 10.
And heads up, I'm NOT budging on these decisions.