Kopa may not be existed in films, but he's in TLK six adventures. I believe Zira killed him when he was friends with Vitani. I think Kopa wasn't in the movie because it would be sad, plus it wouldn't be a good children's movie, if he is killed by Zira, And that's probably why Simba is so overly protective of Kiara.
I know a lot about the KopaXVitani fanfic. But I don't trust it. If Kopa exists, Kopa tries to be friends with Vitani, Vitani and the outlanders try to kill him, Vitani asks forgiveness, Kopa does not forgive. I don't know really who he loved. By the way... does Kopa exist?