I believe Tanaka knew about our Ciel all the time.
Tanaka has always known from the very first day that our Ciel is not the oldest twin. He was not fooled by him; he is helping him! You can see this if you read kuroshitsuji in Japanese and I’ll show this to you.
First of all I would like to talk about the term “Bocchan”. It is translated as “young master” but for me the word “Bocchan” in Japanese denotes much more affection towards the little master than the formal “young master”.
Reading some chapters in Japanese I noticed that Tanaka addresses the true Ciel as “Ciel sama” (Lord Ciel) showing respect for him as the heir and that he uses the term bocchan just for our Ciel. (A less formal and somewhat more affectionate term).
In the same way, I realized that the true Ciel calls Tanaka only by his first name, but our Ciel calls him “Jiya” wich can be translated as Grandpa (again a term that shows affection and closeness to the servant).
Back in that day I believe the two boys were present in this scene. The real Ciel calls Tanaka by his name and the old butler tells “Lord Ciel” to escape. Tanaka is attacked and falls over this boy ( real Ciel) while a hand grabs the other boy.
During the first night in the mansion alone with Sebastian our Ciel still refers to Tanaka as “Jiya”.
Thanks to that BoC side story we saw that Our Ciel calls him “Jiya” since when he was very young.
And Tanaka seems to be happy with this.
And here is the scene of the reunion between our Ciel and Tanaka at the hospital. Tanaka falls to his knees and calls the boy “Bocchan” and our Ciel runs to him calling him “Jiya”. There are no mistakes. Tanaka recognized him as his dear “Bocchan”.
It particularly warms my heart to know that our Ciel received a hug from someone dear to him and that this person really knew it was him and not the heir.
Remembering that it was thanks to Tanaka that Sebastian started to call his master “Bocchan” and so he is called by Finny, Bard and Meirin.
And our Ciel never demanded of them the use of “ciel sama” (lord Ciel) but accepted the affectionate “Bocchan”.
This is so wholesome 🥺
But also makes me question, if Tanaka supposedly knew that O!Ciel was posing as R!Ciel, why would he stay at the manor instead of helping out O!Ciel during the twin reveal?
I also fully believe that Tanaka always knew OC wasn’t Lord Ciel.
As for why Tanaka stays behind? We don’t know his reasons. Perhaps he made Vincent a promise, perhaps he feels that OC has Sebastian (and the other servants) and his duty has to be to RC…
I expect we may learn a lot more about Tanaka once we’re past this part of the manga and heading toward the final arc with confronting the mastermind etc.
