Can we take a moment to talk about...
Undertaker's nails?
I mean, in kuroshitsuji world only demons have black nails
Why a shinigami has black nails? It is only for the character design or it is something more behind?
•Or maybe he painted his nails
As we can see many scars on his body, made me think about maybe he is an experiment like the bizarre dolls. But with a little twist.
He could be like the Frankenstein's monster.
It is a being created from different parts of corpses.
Maybe in the past he have a hard fight and try to heal himself in any way he could. Taking some parts of demons to himself...
I like to think, that undertaker already had a fight with demons at the past, he is so confident fighting with Sebastian and knows exactly how to deal with someone like him. He doesn't even care if Seb is more strong and fast than shinigamis. He can kick his ass again if he wants .
(At this point I'm rambling but still... Isn't interesting theory?)