Real Name: Mar-Vell
Species: Kree
Planet of Origin: Kree-Lar
Joined the Team In: Avengers #89
Powers/Abilities: Super-strength/speed, flight, cosmic awareness, solar energy absorption/projection, wields Nega-Bands.
Who/What Is He: Captain Marvel (or Mar-Vell) was, at one point in time, the cosmic hero of the Marvel Universe. Hence his name: Marvel. As a soldier of the Kree empire, he was initially sent to Earth to spy on its people and evaluate the planet’s threat-level, but he eventually came to be a protector of both the people and planet – before dying, rather humanly, of cancer.
Why We Want Him: He has an interesting story steeped in alien infiltration – which could be beneficial down the line depending on the plots of the next few Avengers films – he has a cool name and costume, and his powers are pretty great. Simple as that.
On the other hand, Thor – while vaguely rooted in Norse mythology – also has super strength, long blond hair, and can fly without rocket boosters, so does the team really need two space aliens that function in essentially the same regard?