Fluffy is the official character of TLK with no gender references. Fluffy is a unisex name that the TLK creators of the film gave it. it's not the cub's final name, Fluffy is seen in TLK 2 trailer for baby Kiara which known Fluffy is the same character as Kiara the cub has gotten a gender while TLK 2 was under production. In the beginning of TLK 2, Timon thinking Kiara was a boy, then Rafiki correct them, "It is a girl".
Kopa on the other hand is only from a few non-canon libros that are not Disney's creation but created por outsiders that did not worked with the Walt disney company which had disney stamp placed on the libros and those libros were only released in the U.S.A. So the answer to this question: "Who's the real one Kopa o Fluffy o both? The answer is: Fluffy. Since Kopa isn't, known he isn't from the movie universe while Fluffy/Kiara is. The truth: Kiara and Fluffy are the same cub.
Like some here have already said, Fluffy was a non-serious name dado to the cub when dicho cub had no real identity.
According to the creators of Simba's Pride, Fluffy is Kiara, and according to the writer of Six New Adventures, Fluffy is Kopa. Kiara and Kopa have never co-existed in anything with Disney's name on it though (Kiara and Kopa being siblings is 100% fanfiction).
That said, Simba and Nala are getting a son in a new disney Junior TLK based franchise! Named Kion, I think? I heard a rumour that Kiara is going to be in it, but I'm not certain if it's true. If it is though, she finally will have a brother in something official!
But yeah. Fluffy is the "real" one if you're going por what the creators of the original film said, but Fluffy can be either Kiara o Kopa depending on which material tu are going by. Both Kiara and Kopa are "real" in their own existences, but Kiara's not real in SNA and Kopa's not real in SP.