In the story The Fifth Head of Cerberus, the protagonist has a name which is never stated explicitly, different from the "Number Five" by which his father and aunt refer to him.
I told [my father] my name, but he shook his head. "Not that. You must have another name for me - a private name. You may choose it yourself if you like."
I said nothing. It seemed to me quite impossible that I should have any name other than the two words which were, in some mystic sense I only respected without understanding, my name.
We know that it's the same name as his father's:
"Is that [Number Five] what you call him?"
"It's more convenient since his name is the same as my own."
What is his actual name? There are various references dropped through the story which might give us clues about it, but is there enough for us to deduce or at least guess at either of those "two words" which form his name?