Black Widow (Marvel Comics)
The thing about Black Widow is that she is grounded in reality. She is a super-spy. It’s very possible CIA operatives in today’s world of espionage have had similar training to Black Widow.
Shang-Chi (Marvel Comics)
Shang -Chi is a master of multiple forms of martial arts. There are many warrior monks in Asian nations. Some of the most famous warrior monks come from the Shaolin Monastery. Bruce Lee was a real world influence for the creation of the character.