What exactly is Tanaka's function in the Phantomhive family? To my understanding from watching season 1 and 2 of the anime, the manga (up to the Phantomhive murders arc) and the OVAs, it seems like he was Vincent's head butler back when he was the head of the house.
But since Ciel has come into power along with Sebastian, his function seems small. In the anime and manga when Sebastian assigns tasks, he always tells Tanaka "The usual will be fine". What does this mean? Can anyone explain?
Tanaka-san mainly only drinks tea. He was the previous Head-Butler of the Phantomhive family before Ciel shows up with Sebastian. Since Ciel makes Sebastian the Head-Butler, Tanaka-san was then task-less and turned into a backup Head Butler. This was later shown when Sebastian had to leave and was unavailable, Tanaka assumed the position of Head-Butler, commanding the staff trio until Sebastian's return.
In the Book of Circus, when the circus members attacked the Phantomhive manor (not knowing that the Ciel Phantomhive they are after are actually in their main camp disguised under the pseudonym Smile), it was shown that Elisabeth was spending the night at the manor. She was awaken due to the noises but Tanaka-san calmed her down and told her that she should go back to sleep while fully aware that there is a broken window (with a dead body IIRC) behind him. Elisabeth didn't notice this since he was there.
This might be his main task now, taking care of Elisabeth when she came over.
In addition he was also the figurehead of the Funtom corporation (anime 1st season episode 9).
Tanaka is basically like a personal guard. Before Sebastian came, Tanaka was the head butler to Vincent. I'm guessing he just exscaped the fire or was out running an errand for him and came back after the fire spread. But after Ciel summoned Sebastian and he became his head butler, Tanaka just moved to an extra butler. Referring to Book of Murder when Sebastian pretended to die, Ciel told Tanaka that he would be his head butler for then on.
I will answer a part of the questions:
Tanaka is/was not only a butler but also a bodyguard, pretty much like Sebastian but of course not a deamon. This in turn means he is pretty strong so he probably just escaped.
From what I understand from the anime and manga Tanaka use to be the Butler until sebastian came along, but since ciel sees tanaka as a grandfather figure ( manga) and the fact that at some point tanaka got injured during the fire ( which is why tanaka doesn't do much) he keeps tanaka in the house. If something were to ever happen to sebastian, tanaka would take his place as head butler.
You need to see Book of Circus.
He, like all the others, helps keep the mansion safe.
In other words, he is also a child killer.
I think Tanaka is quite a knowledgeable person and Butler.
He knows about the many secrets residing in the Phantomhive manor and so perhaps that's why he still remains in the Phantomhive manor as the Butler.
I feel like he would've just retired but perhaps, just like everyone else on Ciel's side, he's hiding some secret. Maybe for himself or of Ciel's. It's quite difficult to explain without spoiling the plot - read the manga, it should be updated tremendously by now and you should probably understand the significance of Tanaka's character.
I might just be making his character seem more important than it is, but all of Ciel's old family and his past in general have some sort of hidden agenda - they all know something that we don't: Elizabeth, her family, Tanaka, obviously Sebastian knows it all, but Ciel's family comes into play and Tanaka was there from the very start of it all.
Although he's ill and perhaps a bit forgetful, he was Ciel's father's Head Butler (which is a huge deal!) It proves to me that he has some knowledge about many secrets, and that he probably acknowledges that Sebastian is a demon and that Ciel isn't who he says he might be...
But he holds Ciel very close to his heart, seeing as though Ciel still sees Tanaka as his grandfather - and he provides him a place to stay even though he has no use to Ciel's "game" (revenge). Sure, he's strong, but he's still old and human.
It's obvious that Tanaka will have a part in Black Butler, maybe not soon - but later it will all come through. If you've watched the anime, it shows him recapping the episodes in one episode and he mentions that Sebastian will go through a "challenge" in the end of it all.
Yeah, this is very early on in the season. But it's a foreshadowing of events. And I'm believing that Tanaka is somewhat the narrator to Ciel's life - as if, he already knows his revenge being played out and that he knows Ciel's biggest fear... and he knows everything because he survives it all (maybe not on Ciel's side...)
