A few things I didn't mention about 9's name...
9 is one of my favorite numbers... its square root is 3 which is also a favorite number of mine.
9 is also how many lives cats have (as we all know)
In Japanese, 9 is pronounced "ku" or "kyu", so in a way, it's a tribute to my beloved, Q.
My first degree was in mathematics, so to name him with a number is kind of fun and very much 'me'. After all, Q was originally Diff EQ, from differential equations, the class I was taking when I got Q.
A friend asked me if I wound name my next cat, "Pi", being the geek that I am, and I would have used the Greek letter, not the Roman alphabet for his name. However, that would have been a bit too much like the artist who was formerly known as Prince...
So, there you go, the story of 9's name.