Hi there OP! Well Kopa was Simba's son in a TLK book series known as '6na' (Six new adventures) which tells about stories that never happened but most fans believe the many things such as the water buffalo giving Taka (Scar's original name) his scar. There's also many other different characters such as Mufasa and Scar's parents, An evil snake, Mega, Leo. It's a very wide universe in those books and though I haven't read them myself, I've looked it up and it's a good series and can also be a sort of 'Sequel' to TLK.
Many believe that Zira who was vengeful after Scar's death and murdered Kopa, explaining his absence during SP and the reason for the Outlander's exile as well Simba's over protectiveness of Kiara.
When I think of what that BRUTE did.. I get a little TENSE!