User blog comment:B214/DxD Queen tournament/@comment-13032946-20150209034619/@comment-93.105.240.132-20150212124332

Ok the simplest way to answer this is through death.

You have to look at a character and ask yourself "could the series continue without him/her?"

So could the series continue without Rose? Yes it could. But could it continue without Issei or Rias? No!

Iseei is the protagonist, while Rias is the source of his power. If Rose died however the series could continue.

In a way the series could continue even without Issei but that would cause a drastic change into the series.