I couldn’t sleep last night and somehow this popped into my head.
I think that Grell and the Undertaker are either related to, or alternate dimension versions of Death Scythe (Spirit Albarn) and Stein.

The Undertaker and Stein are both obsessed with death and life, and things that are pretty morbid. They both have grey hair and a scar covering a large portion of their face.

Grell and Spirit both have luscious red locks (although Grell’s hair is longer), both behave extremely inappropriately and perversely.

Maybe Spirit and Stein really are descendants of Grell and the Undertaker, because Black Butler was set way before Soul Eater, time period wise.

I’m sure I’m not the first to notice this parallel between characters in Black Butler and Soul Eater, but there’s my two cents.