It depends by what you mean by "real".
Is Kopa real as in he is a character found only in
The Lion King: Six New Adventures and some German audio tapes? Or he is real as in is he is an existing film character?
He is a real character that appears in the
Six New Adventures novel series, and here's a picture of him:

However, Kopa only exists in that particular book universe. He was indirectly denied by the original filmmakers on the audio commentary when they named the cub at the end, "Fluffy". Therefore, he is not the cub held up at the end so he is not an existing film character.
Fastforward some years later, Phil Weinstein (a storyboard director on
The Lion King II) confirmed in an interview that the production crew had no knowledge of Kopa during the making of film. The full transcript of the interview can be found
here. However, here's the main excerpt of what was said:
[quote="Phil Weinstein interview"]
Aniu: Are you familiar with Kopa, the original son of Simba and Nala? And if so, was there a reason you all
opted not to put him in The Lion King 2?
Phil: No…what was his name?
Aniu: Apparently his name was Kopa, and it was—I forget exactly where the fan knowledge comes from, because I’m not—
Skul: [interrupting] I can answer that. Kopa was the son of Simba and Nala in a series of books that were
published immediately after
The Lion King. And there were six books total, all of them 100-‐page kids books, but they all had Kopa in them.
Phil: Wow. No, I wasn’t aware of that.
Skul: Yeah. A bunch of people have been wondering why it was Kiara instead of Kopa in
Lion King 2. And, I guess not even you know the answer.
Phil: No, you’re going to have to track down—gosh, I can’t even remember. Who was it…was it Darrell
Rooney that directed
Lion King 2? Is that right? I think he was the one who ended up taking it to the finish
line.
Aniu: Okay.
[Note from Skul: Yes, Darrell Rooney was the director]
Phil: So you’ll have to find him or one of the writers and ask them. They might know.
Aniu: Okay. We’ll definitely have to track him down and eventually ask that question.[/quote]
Someone named KieranTheWolf (who I later found out to be SuperBabySimba, who was once a user here) contacted Alex Simmons, Kopa's creator, via email. Simmons admitted he was never contacted or involved with the sequel's filmmakers at all:
In conclusion, Kopa is not an existing film character as he was never known onto the filmmakers therefore no hint of his existence is made in the film universe.
The Lion King II makes it clear that Kiara is the cub at the end of the film, and in the end, she became "Fluffy". However, he is a real character that exists in an alternate universe where Simba has a son.