"Who IS Captain Marvel?"
Well, that's a tricky question to answer, as there have been over a half-dozen of them in the Marvel Universe.
- The original Captain Marvel was Mar-Vell, a Kree warrior-turned-Earth's Protector.
- After Mar-Vell's death, Monica Rambeau gained super powers, and took up the name.
- When Mar-Vell's son Genis-Vell decided the code-name Legacy wasn't cutting it, Monica agreed he should have the mantle. Monica took up the name Photon until Genis wanted that name, too. Monica took the name Pulsar.
- When Genis went temporarily insane, his sister Phyla-Vell took up the mantle of Captain Marvel, until eventually changing her code-name to Martyr.
- Khn'nr was a Skrull sleeper agent, who believed he was Mar-Vell transported to the future before his death. He died in battle against the Thunderbolts.
- When Norman Osborn created his team of Dark Avengers, Noh-Varr, formerly Marvel Boy became the new Captain Marvel, but only until the team disbanded when Osborn was imprisoned.
- Currently, Carol Danvers is Captain Marvel, having gotten her powers from Mar-Vell in an accident involving the Magnitron.
- Oh, and there's the Captain Marvel who lives in Fawcett City, but he's not a Marvel Comics character, so we won't talk about him.