Sebastian couldn't say his name.
The boy was always "Young Master," "My Lord," or "Sir." Even when he was ordered to work in disguise around the child, he effortlessly avoided saying the boy's name, unconsciously opting instead for titles such as "Earl" or "Lord Phantomhive." Said words were already in his everyday vernacular, and he was willing and able to use them to address his young master, even undercover.
The other servants would say his name, presuming of course they put some sort of acceptable title before it, like "Master," or after, like "Sir," but not him. When he was alone and all he had to keep himself company were his thoughts, he addressed the boy as "Young Master." In moments of crisis when he called for the boy in panic, the child's birth name never grazed his tongue. It was always "Young Master."
Names were how humans identified themselves, but in truth, he didn't really care what his contractors' names were. They were all just the next delicious meal, and their name didn't change that.
None of his contracts had ever lasted this long, however.
At first, he assumed it was his own aesthetics that led to this situation. A butler shouldn't say his master's name, after all, and he imagined that that had seeped into his unconsciousness as he lived up to his role as the perfect butler, demonstrating his obedience to demonic ways. But that wasn't it.
He simply couldn't think of him as his name. He only thought of him as his young master.
"Hm. I supposed he does have a name." Sebastian smiled. "And that name is 'Young Master.'"
The jingle of a calling bell halted his thoughts. Sebastian sighed. "Really, you can be so demanding sometimes… Young Master."
A (very short) drabble that's been on my mind for awhile. I don't think Sebastian is capable of saying "Ciel," let alone thinking it.
I always appreciate your thoughts, dear reader. :]