It has been said that, if you're thinking of what to name a child, then the best thing to do is to walk into the middle of an open field and shout the name at the top of your lungs about six or seven times. If you still like the name after the seventh time, then you can keep it - after all, you're going to be shouting it at the top of your lungs for the next twenty years.
People who name their children "Tidus", "Ivana" and such therefore puzzle me. Not only would the names be terribly impractical for those times in which the parents would have to shout them at the top of their lungs, but they would needlessly burden their owners with years of playground taunts. Ever made fun of that kid with the weird name in your kindergarten class? Well, congratulations - you probably just scarred that kid for life. That's one aspect of your current lifestyle that your son or daughter will never understand once they've grown up and moved on.
"Tidus... that's a strange name. Why did your parents name you 'Tidus'?"
"I don't know. He was some character in a video game my dad liked, I think."
A similar approach applies even to fictional characters. Unlike our children, we can name our characters however we want, regardless of established naming conventions or even linguistic phonics. Like our children, however, we have to realize that once we release those names out into the wild, then they're going to be completely out of our hands. Any mistake we make in the naming of our characters, we have to live with for the rest of our literary lives. There are no nicknames where we stand.
I once read an amateur fantasy story where a racially diverse group of adventurers (think human-dwarf-elf-hobbit) were on a quest to retrieve a magical sword/scroll/jewel/cheeseburger. In order to do so, however, they first had to free a mighty hero from his icy prison in the frigid wastes - the last survivor of the original party that went hunting for their intended prize. The adventurers battled their way through groups of monsters, finally melting the ice that held its prisoner in check. And that was when the erstwhile narrator revealed that the hero's name was... Sir Jeff.
I don't have the paper any more, mind you. I handed it back to the author, and advised him to either change the names or burn the whole thing. Abominations like that shouldn't be allowed to exist on their own.
And all this, because of a single name.
Each name, you see, carries some baggage to it. When we hear the name "Jeff", a certain image always comes to mind - an image that plays a vital part in forming our first impressions of the subject. Exactly what "Jeff" looks like to each of our imaginations depends on who we are and how many Jeffs we've run into. His hair may be jet-black or rust-brown... maybe even blond, depending on who we know. He may or may not have a mustache.
This is why names like "Jeff" can be appropriate for some settings but not for others. You can give that friendly neighbor of yours the nickname "Jeff" because he's as neurotic as Jeff Goldblum, or as blue-collar as Jeff Foxworthy, or simply as funny-looking as Jeff in Mutt and Jeff. But you can't give that venerable paladin hero among your fantasy characters the name "Jeff"... because those modern connotations don't - and shouldn't - necessarily apply to fiction.
Let's try an exercise, then. I give you a name, and you tell me what impressions come to mind. You don't tell me if you know anyone by that name - you tell me what impressions first come to mind once you hear it. Ready? Good.
When I think of "Cassandra", the most obvious impression I get is one of black hair. Long, straight, glossy black hair, I suppose, but that's probably my personal imagination at work already.
I wouldn't be surprised to find that your impression had black hair involved as well. There are few people out there who can claim the rare privilege of knowing a blond Cassandra, after all.
This is how a reader will see the characters that are set before them. The characters almost never get a chance to properly introduce themselves to the reader, and so there are no such "first impressions" that we normally get in human-to-human interaction. There is only the name, and in a story the name gives most of this first impression.
Be careful about your children, everybody - whether they're your biological descendants or merely the products of your overactive imagination.Your names, after all, will eventually define them.