Personally, I think Kopa got in some sort of accident along the way after the first
TLK. Clearly, at the end of the first film, the cub Rafiki held up resembled more of a male cubby. ^^ It didn't look female to me and thus, I believe it had to be Kopa. I know Kopa isn't mentioned in the movie, as if we all knew that already.
Kiara also wasn't mentioned in the books that he appeared in so there had to be some sort of time period where Kopa existed before Kiara. In regards to what possibly happen to Kopa, I think he ventured off by himself or some sorts and got killed, this can pretty much explain why Simba is overly protective of Kiara.