Omg i also thought thesame!! Ever since that arc wjere he left the locket with claudia phantomhive on it i was already thinking that maybe they had a past together and since understaker was a shinigami she died and he was left alone...or maybe he killed himself and became a shinigami....it makes sens if hes related to ciel because hes been sticking around both vincent and ciel for who knows how long!!
The way Ceil is able to sence the shimigami is also a hint, I think.
http://www.mangago.me/read-manga/kuroshitsuji/an/kuroshitsuji/103/pg-4/
And in the page following that, the shinigami mention that it could be because of his lineage
http://www.mangago.me/read-manga/kuroshitsuji/an/kuroshitsuji/103/pg-5/
There has gotta be a shimigami some where there...
Ok is it just me who came up with this crazy conclusion, is there someone else as well!?
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Since the ship arc I have this crazy idea that Undertakeer is Ceil's GRANDFATHER!!!!! This recent chapter is making me think I might be right!
What do you guys think?