Well, I personally still believe in the theory that Kopa had been killed by Zira, which caused Simba’s greef and his overprotective attitude towards Kiara. And which btw resulted in Zira's banishment (exile).
But I am open minded towards other theories, too.

Maybe Kopa only existed in the first movie, when the creators did not even think about making a sequel. When they finally did, they had to replace Kopa with Kiara in order to keep the story going as the idea was to create a love-story where two families antagonize and the bond between the two main characters finally end this conflict. Kovu had to be male, in order to be seen as the successor of Scar (I think) and Kiara had to be a female, as a male-male relation would have led into a severe social discussion about a family movie. ^^