The steward of the Phantomhive household. No one actually knows what Tanaka does around the house, save for sit around and drink tea. His reaction to any situation, no matter how strange, is to merely smile, take a sip of his tea, and chuckle. In other words, he's a bit of an enigma. Most of the time he is in what is known as a chibi form. It is said he can only revert back to Normal Tanaka for a few minutes before reverting back to his small form.

Spoiler

It seems Tanaka served Ciel's parents as well. When Lord and Lady Phantomhive were murdered, the young Ciel ran to Tanaka, who warned him away. Tanaka was then stabbed by an unseen assailant...