The Undertaker (アンダーテイカー, Andāteikā) (His real name is not confirmed yet,) is an former Grim Reaper (Shinigami) and a funeral director who works with the underworld of Britain. He frequently acts as an informant to Ciel Phantomhive.
In the early days of Black Butler, the Undertaker was a somewhat harmless old man who would trade underground information for what he considered 'first rate laughs'. He was neither good nor evil, he was neutral as all Grim Reapers should be. Undertaker was and is still regarded as a legendary Grim Reaper in both the anime and manga. In the anime, he passed judgement on the soul of Robin Hood and sent Marie Antoinette to Hell, this testifies to Undertaker's age and his ability to judge mortal souls. It also seems like it is very hard to upset Undertaker, He wasn't even angry when Grell buried him in salt.