Monica Rambeau can transform herself into various types of EM energy. These days, she's called Spectrum. Before that she was known as Pulsar and then Photon, but originally she was Captain Marvel. With the Captain Marvel name being claimed by a bunch of people over the years, Monica was unfairly forced to come up with something new. She made her début as the captain in a Spider-Man comic in 1982 but quickly joined the Avengers the following year, eventually becoming its leader. Throughout her career, in a variety of comics, she's been consistently depicted as capable and a good hero. She sets a great example. Why she won't be in the Avengers movies: Let's be subtle here and ask, "How many black people do you see in the Avengers?" Yeah, the Falcon, and that's it. Fine, the situation will get a bit better with the introduction of the Black Panther. But let's ask another question: "How many women do you see in the Avengers?" Yeah, Black Widow and (now) Scarlet Witch. At this rate, it'll be the twenty-second century before the MCU sees someone who's black and a woman in an Avengers film.