Doctor Strange would be an interesting character to focus on in a Harry Potter MCU fanfiction. He already has knowledge of magic in the MCU, but it's a different kind of magic compared to that in Harry Potter. He could compare and contrast the two magics, and perhaps even learn new spells from the Harry Potter universe. His presence could also bridge the gap between the two universes in terms of magic.
One could argue that characters like Harry Potter or Sherlock Holmes are among the most famous. They have captured the imaginations of millions worldwide through books, movies, and various adaptations.
Cervantes 'most famous character was Don Quijote in Don Quijote. Don Quijote was a wealthy nobleman, but because of his vanity, stupidity, and greed, he eventually fell into a meaningless knight's adventure. The story of Don Quijote showed Cervantes 'deep insight and criticism of Spanish society and culture, and also provided important enlightenment for later literature and art.
Cervantes 'most famous character was the male protagonist of Don Quijote, Sancho Panza. Sancho was a wealthy knight, but he fell into an adventure in pursuit of love and freedom. He tried to save his village against the windmill and the knights, but failed. This story reflects the social and cultural background of Spain in the 18th century through the fate of Sancho, and also explored the theme of freedom, love, honor and so on.
Batman has to be up there. His dark and brooding nature, along with his gadgets and detective skills, have made him a favorite. His no-superpower approach makes him stand out in the comic book world.