[quote="Amanda"][quote="Pixielicious"]WHY is Kopa so popular. He's from a book, has very little background and is quite 2 dimensional. Kiara is better because she has a personality/voice/story/life.[/quote]
Because he's the only alternative to Kiara.

[/quote]
No he's not. There's also Fluffy who, if not seen as Kiara, is the only other official cub next to Kiara and thus makes the only even relatively fair rival. But it would be just as unfair because it's even more free for a fan's imagination than Kopa is as it has no real name and gender not to mention a story.
I still think it's silly to ask this kind of question. You might as well ask "Do you prefer someone else's character to a character you can imagine any way you ever want?" Of course the own imaginations are gonna be more popular.
I personally prefer Kiara to both Kopa and Fluffy (who anyway is the same as Kiara in the general official matters of the film universe.)
1. We already saw a story about a prince. Princess gave a nice change in point of view and story aspects. So I wouldn't imagine Fluffy as a boy anyway.
2. The princess they created certainly gave new blood to the line of royal children in the films and to the story. I'm not a big fan of Kiara and I do find her annoying, but exactly because of those annoying characteristics, she's a good character fot the entity of the story. As in, refreshing while still in some ways familiar. Resulting in Kiara was well written in those terms.
3. I find it fascinating that the royal bloodline of Pride Lands no more stuck with only Mufasa’s blood but was mixed with a new male’s from outside the pride. It might make the future generations more and more interesting. Of course Nala wasn't Mufasa's child, so there already was someone else's blood in Kiara... But still. I mean through the king.