
Kopa in particular is pretty controversial in himself, since he is very popular but doesn't have a place in the canon universe. According to the filmmakers, Simba never had a son. That is canon and you really can't argue with that (saying this because to me it looks like there are people who do). They considered giving Kiara an older brother, yes, but officially there is only Kiara. Only Kiara. And those are the facts. Now wait with the pitchforks and torches until I'm done. See, you can decide to not care but at least be aware. (hey, it rhymes)
Kopa is "semi-canon". Along with Ahadi, Uru, Mheetu, Bhati, Ni, Kula, Chumvi, Tama, Zollo, King Joe, Rafiki's fat cousin/relative, Pumbaa's possible girlfriend, Mufasa's monkey assistant (whose names escape me) and all the other named or unnamed characters who have appeared in Disney concept art, books or comics. They did not appear in the films and only the films are canon but Disney still technically owns these characters, thus semi-canon.
If I read what SBS posted about the TLK6NA author is correct, he could very well be a fanfiction writer who was lucky and got his work published by Disney. But we don't know that either. It's possible, but we don't know. Or actually, now that I think about it, any TLK-related stories or characters created after the release of the first film could technically be fan made. Because the people who made those things must've at some point thought "Oh, yeah, The Lion King. I quite like that film. I think I'll make a [comic/book/drawing/sequel film] for it." (more or less xP), alternatively "Oh, yeah, The Lion King. People quite like that film. I think I'll ask someone working here on my Disney magazine/children's book publishing company to make a story for it to earn some money." and some were lucky and got recognised

Aaand for the record, I like Kopa C: I'm with the general fandom's opinion that a son of Simba existed and that he disappeared because of Zira and that this is the reason for Simba's behaviour in SP. It really makes so much sense. Much more sense than "I will banish you just because you like Scar!" xP
Edit: I think I contradict myself at least once here. Oh, well, it's already a long enough opening post, more food for discussion xP