I have seen in the Kuroshitsuji manga that the Undertaker has a caring eye on Ceil. He also has a death locket of his grandmother. Any ideas of the reason why he cares so much for Ceil.
I've read the manga up until volume 16, so I'm just taking a wild guess here, but… I think it may be due to the Undertaker's relationship with Ciel's father, Vincent. During a flashback in the Noah's Ark Circus story arc, the Undertaker can be seen with Vincent and a few others (including Lau, I believe), playing a game of pool and discussing "work".
Perhaps he made a promise to Vincent to keep an eye on Ciel, but that would be predictable, wouldn't it?
I have been keeping up to date with the new release of each chapter of the manga on online readers. I just read chapter 103. The German shinigaumi says something like humans shouldn't be able to see us, but because he has a contract with that demon he has one foot in the grave…the other one says some thing like maybe it's because of his bloodline. I'm thinking now maybe Undertaker is perhaps Ceil's grandfather? That would make sence to me by how much he cares for him.