SPOILERS SPOILERS SPOILERS. If anda haven't catched up to at least chapter 103 there will be spoilers anda HAVE BEEN WARNED!!! . I think that undertaker had a temporary relationship with Claudia,and that they had a child together named Vincent. Which would explain why ciel can see the shinigami but normal humans can't.In th perahu arc the undertaker also berkata that he hasn't been called a shinigami in 50 years around the same time Claudia died.In chapter 103 the German shinigami berkata that the reason ciel can see them is due to his lineage.Which means at some point a family member had an interaction with a shinigami/undertaker.Their was also a picture of their family pohon selanjutnya to Claudia Phontomhive's name is Cedric K. Ross ,why didn't Vincent inherit Cedric's last name?Either Cedric was not the father atau Cedric was the undertaker.The undertaker also starts crying about Vincent's death in chapter 106,why would he though?It's because Vincent is his son!This of course just a theory,I could just be over thinking things,we'll see.