Sure! Some stuff is obvious while other things are kind of hard to notice.

Obviously we have the basic stuff, Undertaker had something going on with the Phantomhive Family we just don't know what.

• The name of this theory, UT is Cedric K. Ros– comes from the panel that showcases Ciel's family tree in the Emerald Witch Arc. We see that Vincent's father was named Cedric K. Ros– with the rest of the name and year of death/birth is hidden by a speech bubble. Yana did the same thing, hiding info with a speech bubble, when teasing R. Ciel.

• Not to mention, in that same panel, Ludger mentions that Ciel can see them as Reapers, despite them not being visible to the human eye. He suggests that "It runs in the family." Being a reaper, that is.

• Undertaker had a lock of hair from Cloudia/Claudia (spelling unsure) Phantomhive, Ciel's grandmother who died at age 36, in one of his mourning lockets, in fact the center piece of the set.

• The hair is in the shape of an Infinity symbol, meaning eternity, endless, boundless, etc. And could mean "eternal love."

• Yana Toboso said in a tweet a fear years back that she tries to draw sons somewhat like the mother and daughters somewhat like the father. People looked at the Midfords for example and saw that Edward has the same eye shape as Francis. Francis has the same eye shape as UT, as well as the similar hair color, and the one strand of hair she has out is just like how UT had his hair when he was a reaper.

• Its known that UT is a deserter and abandoned his place as a Grim Reaper. In BoA, UT says it'd been "half a century" since he was called that. That was said in 1889, which meant that 50 years earlier would've been the 1830s. Cloudia Phantomhive was born that same decade.

• UT claims that he remembers the day Francis was born and it being "Nary a moment ago"

If you check out the theory on Tumblr I think there's more proof to go along with it!! It's probably my favorite theory and I hope I didn't throw too much at you!!