There are many terms of endearment used. Thousands of others who response to all of the same ones. There is only one Sypha, same as there is only one Trevor. [There's a terribly large amount of Trevors, but that's not the point.]

Your names are yours and yours alone. Why would I use something else that is not fully you?

[At the end of the day, he cares about them being them. They mean the world to Alucard, he's shown that in all ways great and small. But they are unto themselves, people in their own right, and that is something too much of the world around them seems keen to forget or ignore. (Pets. He hates it.)]