Black Widow was one of the first people to join the Avengers, being a key member of the Initiative since before Iron Man 2. She was the only official member, soon followed by Iron Man, Captain America, Hulk, and eventually Hulk and Hawkeye. However, some people like the Disney Corporation likes to keep her out of merchandise and without solo films.
Black Widow is one of the best characters in the MCU and one of the best Avengers. However, she doesn't get to show this more often than the remaining Avengers who get their own Solo Movies and Sequels. Black Widow in the comics shows a very powerful member of the Avengers, and one of the most powerful characters in the Marvel Universe.
Remember that time she used Mjolnor?
Yeah, I know that you are all going to say something along the lines of, "But Widow didn't even try to lift the hammer in Age of Ultron?" She didn't at all, but there is a reason for this. In What If: Age of Ultron #3, she is left with no choice but to use the fallen Mjolnor and....
She becomes a god. Black Widow is the only Avenger, other than Thor, existing in the MCU who can use the hammer. That's pretty awesome, seeing as none of the others can do that.
She's almost as old as Captain America
Surprising, I know. Black Widow's official birth date is November 22, 1928, which means she was four when Captain America got his powers. She would be 77 years old in 2015.
The reason for this is that Black Widow was been enhanced with biotechnology, making her body resistant to aging and disease. Black Widow also has white blood cells that can fend off any foreign element or microbe from her body. She can do anything that she puts her mind to, using her intellect and her psychological assessment of people to her advantage. She is definitely not a damsel.
She can kick some major ass
She doesn't need a shield, a suit, a hammer, or an anger problem to defeat bad guys. Black Widow took down an entire team of HYDRA spies on her own, a whole team of Hammer Body Guards on her own, and could partly hold her own against the Winter Soldier.
She's also one of the strongest Avengers, mentally as well. I wouldn't have been alright as she was if I went through Soviet brainwashing. She was taken from a young age, trained in ballet and murder, forced into being unable to have children, and made to kill without remorse. No ordinary person would be able to stand up from that and fight for good.
I mean she's the Widow
What else would you expect? She's been known to kill and to fight for justice. When deception is afoot, Black Widow will save the day.
That is if she will ever get through the glass ceiling.