Although several characters became franchise mainstays, most prominently Professor X, Magneto, and Wolverine, many others were introduced without plans of how they would be developed.

Source: https://screenrant.com/mcu-avoid-fox-x-men-mutant-trend-avoid/

As the X-Men fought their way through Apocalypse's citadel, Magneto was able to escape and fight back against the dread lord.

Source: https://www.cbr.com/lucky-xmen-victories/

Can you see him as Magneto?

Source: https://comicbook.com/movies/news/marvel-fan-art-imagines-jason-isaacs-as-the-x-mens-magneto-for-the-mcu/

Created from the consciousness of Professor X and Magneto, Onslaught is a formidable psionic entity.

Source: https://screenrant.com/13-avengers-villains-more-powerful-than-thanos-ranked/

For years, Magneto, Sabretooth, Mystique, and others have been on and off the team, sometimes leaving violently.

Source: https://www.cbr.com/xmen-newcomer-struggles/

Her status as a mutant and the child of Magneto made a huge difference in her popularity, but the Avengers' '90s popularity slide affected her as well.

Source: https://www.cbr.com/popular-marvel-characters/

Magneto admits to himself that he still struggles with what he has done.

Source: https://screenrant.com/magneto-villain-act-xaviers-wolfsbane-new-mutants/

Magneto joined her and they formed the Brotherhood of Arakko to protect its people.

Source: https://www.cbr.com/sins-of-sinister-essential-xmen-comics/

Magneto started life as a villain, but all of that changed after Scarlet Witch depowered the majority of the mutant race.

Source: https://www.cbr.com/ruins-marvel-superheroes-dark-reboots-potential/

Magneto survived by using his powers to keep his blood pumping, surviving the battle.

Source: https://www.cbr.com/marvel-comics-popular-villains-best-victories/

Marvel Legends X-Men '97 action figures add Magneto, Nightcrawler, Jean Grey, Goblin Queen, The X-Cutioner, and Cyclops.

Source: https://comicbook.com/marvel/news/marvel-legends-x-men-97-action-figures-wave-2/

Their roster included Quicksilver and the Scarlet Witch and they were led by the previously-established villain, Magneto.

Source: https://www.cbr.com/major-superhero-powers-created-by-retcons/

Then Magneto decided to intervene.

Source: https://www.cbr.com/xmen-greatest-victories-marvel-comics-2018-2023/

This came in the form of X-Men: First Class. The film showed how the team was formed and how Professor X and Magneto became friends (and later enemies).

Source: https://gamerant.com/best-marvel-movies-not-part-of-mcu-ranked/

This led to a retaliatory strike from Storm and Magneto, one which cost the Master of Magnetism his life but ended with Uranos's defeat.

Source: https://www.cbr.com/xmen-storm-genesis-arrako-civil-war/

While Magneto and Professor X have been long-standing enemies in the pages of comics, they were always really on the same side, just to different degrees.

Source: https://screenrant.com/magneto-forgotten-brother-replaces-xavier-perfect-xmen-nemesis/