Ciel does not see Sebastian as his father.
He is actively repulsed by the idea and Yana has shown this explicitly at least twice: once in the Circus Arc and again in the Murder Arc.
I know there’s a significant segment of the fandom who genuinely believes that Ciel wanted to have Sebastian to replace his dead father, but I disagree strongly, not only because of the above, but also because Yana has shown us that OC was much closer with Tanaka than Vincent.
Besides, if you look at the revisited contract scene from ch 138, it almost seems as if Ciel is not thinking of replacing Vincent when he makes the decision to accept Sebastian’s offer, but in fact is looking for a new protector to replace RC, his dead twin.
It’s impossible to talk about this moment without the gorgeous full-page series from the chapter.
Sebastian offers the contract. Ciel recalls his brother promising to protect him. He gazes out at his brother’s corpse and realizes that his only way forward now is through making a pact with the demon. His brother’s soul is gone already, as is his life.
So Ciel becomes his twin. He takes Sebastian on as his new shield for his revenge. Sebastian may be someone that Ciel relies on, but he is absolutely not his new father figure.
