In Captain America: The First Avenger, the Red Skull has nothing but contempt for Adolf Hitler and his ideology, seeing him as nothing but a convenient backer to be discarded. Likewise, it's implied Hitler effectively exiled Schimdt because of his disfigurement. This is a MASSIVE change from the source material where Hitler was a mentor and father-figure to the Red Skull, who remains a fervent Nazi because of everything Hitler did for him. Hitler was also the one who conceived the Red Skull persona for Schmidt and gave him his iconic mask.
In the comics, Steve Rogers and Bucky Barnes don't meet until after the former is given the serum. They're still best friends, but there's a clear sense of seniority due to Steve being noticeably older than Bucky, serving as both a role model and mentor with Bucky as his Kid Sidekick. In the original 1940s comics, their dynamic was very similar to the father/son-but-not-quite relationship between the contemporary Batman and Robin, but modern comics (read: Ed Brubaker) shortened their age gap to four years — giving Bucky the Age Lift that made him more of a partner, but still preserving the power dynamics. In the movies, they're closer in age and childhood friends, having a brotherly relationship between equals. This change also has the added benefit of giving skinny Steve someone who has always believed in and stuck by him. "Even when I had nothing, I had Bucky," was not something Steve could have said in the comics, no matter how much he values his friend.
In the comics, Hawkeye is Heterosexual Life-Partners with Captain America, due to Cap giving him a second chance as an Avenger, though they do give each other a hard time every now and then. He's often not keen on taking direct orders, and the only one he'll follow without question is Cap himself. In the movies, this is greatly lessened. While Hawkeye does have a respect for Cap, enough that he sided with him in the Civil War, they aren't portrayed as being close friends like they are in the source material.
Bucky and Sam Wilson's relationship is much more antagonistic than in the comics, with the two always making jokes at the other's expense and constantly at each other's throats off the battlefield, and are pretty much only connected through their mutual friendship with Steve. It isn't until The Falcon and the Winter Soldier that they start to build a friendlier and more relaxed dynamic. In the comics, the two get along very, very well even outside of both knowing Steve, and don't ever really make fun of the other.
Howard and Maria Stark are his biological parents in the MCU. In the comics, he is actually their adopted son. His real parents were two S.H.I.E.L.D agents, his biological father being a mole for HYDRA, until he was killed by Tony's biological mother upon discovering his true colors. Tony's mother then asked the original Nick Fury to ensure her child's safety. After being tossed into an orphanage as per S.H.I.E.L.D. protocol regarding unwanted pregnancies, Howard Stark learned of the baby due to being an associate of Fury and in response to the events surrounding their firstborn son Arno Stark, Howard and Maria decided to adopt Tony and the rest is history. That said, these were after the MCU started, so it was originally true to the comics at first.
In the comics, Pepper Potts and Happy Hogan are married while in the MCU, Pepper becomes Tony Stark’s main love interest. In the comics, Tony has a lot of different relationships, with him and Pepper only having a brief fling, and unlike MCU-Tony, he never settled down and started a family.
The Avengers: Tony Stark / Iron Man and Bruce Banner / the Hulk become friends in the film. Not only are they both experts in science, Tony recognizes something of himself in Bruce, in that they're both men Cursed with Awesome that could use their terrible powers for good. In the comics, the two aren't friends, and Tony's "science pal" is usually Reed Richards. Tony was even one of the men that exiled Bruce from Earth in the prelude to Planet Hulk.
In the Ant-Man comics, Scott Lang spent much of his comics time as a supporting character for Tony, being sort of like his sidekick. Iron Man's most famous comic book story, Armor Wars, had Scott play a pivotal role. Here, he doesn't even meet Tony until Captain America: Civil War, which put them on opposing sides no less, and, ironically, Tony doesn't even know who Scott is. He only gets to act as Tony's sidekick in Avengers: Endgame during the Time Heist, when the two are tasked with retrieving the Tesseract.
In the comics, Hank Pym is a scientific peer and close friend of Tony Stark. Here, the two have never met, and Hank holds a grudge against the Stark family for how Tony's father Howard attempted to secretly replicate his research. It isn't until Tony sacrifices his life to defeat Thanos that Hank finally lets go of his hatred against the Starks.
Tony becomes a stern father figure to the young Peter Parker and is essential to his growth as Spider-Man. In the comics, Spidey didn't have such a relationship with Stark. However, Tony did have such a relationship with Peter in the Ultimate imprint, and they were friends in the lead-up to the 616 Civil War (they had a falling-out during when Peter defected to Cap’s side).
Another major change is Tony Stark's relationship with The Mandarin/ Xu Wenwu, or as he is known as in the MCU. In the comics, The Mandarin is Iron Man's Arch-Enemy due to the former supporting the rebels who captured Stark, causing the latter to build the first Iron Man armor. While Iron Man largely follows this (by having The Mandarin's terrorist organization, the Ten Rings, kidnap Stark), and Iron Man 3 has Stark fight an impostor Mandarin by the name of Aldrich Killian, Iron Man ends up dying in Avengers: Endgame without ever encountering the real Mandarin (Xu Wenwu), or even knowing the latter's existence for that matter.
The Incredible Hulk: In the comics, Rick Jones is one of Bruce Banner's and Hulk's closest allies and in fact was indirectly responsible for the Hulk's origin, as Bruce was caught in the fateful gamma bomb while saving his life. In the MCU, however, the Hulk's origin is based on the Ultimate and TV Series, with Bruce transforming into the Hulk while experimenting on himself with the Super Soldier Serum, while Jones is a mere acquaintance of Bruce, being a student at his college.
The Asgardians, already overhauled and remixed from Norse Mythology, are altered even further in the MCU. Heimdall and Lady Sif are not siblings, Hela is Odin’s daughter instead of Loki’s, Frigga is Thor's biological mother rather than step-mother, and Donald Blake and Thor are two different people, the former being Jane Foster’s unseen ex-boyfriend.
In the comics, Maria Hill is Nick Fury's successor as director of SHIELD and a decided foil, with what little interaction they have being frosty at best. In the movies, she's his loyal Number Two.
In the Black Panther comics, Queen Ramonda is T’Challa’s stepmother and Shuri is his half-sister. In Black Panther (2018), they are his biological mom and full sister respectively.
Carol's best friend is a fellow Air Force pilot named Maria Rambeau. In the comics, the two have little to no history with one another, and it's actually Maria's daughter Monica who is Carol's ally. This is because Carol's usual best friends, Spider-Woman (aka Jessica Drew) and Jessica Jones, cannot be used in the film — the film rights of the former belong to Sony instead of Disney who runs the MCU, while the latter starred in her own series, with her ties to the superhero community Adapted Out.
Goose is Carol's pet in the comic, but in the movie she's the original Mar-Vell's, and later stays with Fury, while Carol remains mostly indifferent to her.
Natasha and Yelena never trained together in the comics, as Natasha is several decades older than Yelena; Yelena was also intended to be Natasha's Superior Successor, but ended up as an Evil Counterpart to her. Here, Yelena is only five years younger than Natasha and they were reared as sisters for a few years, trained at the Red Room in the same timeframe, and are ultimately allies.
Melina Vostokoff, known as Iron Maiden, and Red Guardian were villains in the comics, with very antagonistic relationships with Natasha. Melina was defined by living in Natasha's shadow and jealous over it, and Red Guardian was even Natasha's husband at one time! Here, partly to account for Natasha being aged down significantly, they are much older than her and not antagonists. Instead, they're parental figures who raised Natasha and Yelena as children for an undercover mission under General Dreykov and came to genuinely care for them, but were essentially forced to turn them over to the Red Room. However, they still love them as their own daughters but have trouble showing it until helping Natasha and Yelena destroy the Red Room allowed them to reconcile.
Alexei Shostakov was Natasha's husband in the comics, his presumed death making her a literal widow. This aspect is transferred to Melina in the movie, and he's more of a father figure to Natasha.
In the comic books, Taskmaster has no personal connection to Black Widow or the Red Room program, with her role in the movie being more traditionally associated with the Winter Soldier. This version of Taskmaster's origin is intimately tied to Natasha's morally dubious past.
Eternals: Sersi and Makkari had an on-again off-again relationship in the comics prior to the latter's Gender Bender. Here their dynamic is not particularly emphasized (although they are clearly comrades) and Sersi and Ikaris have the "it's complicated" relationship status instead.
In the comics, Cassie Lang spent most of her childhood with her father after her parents divorced, and resented her mother and new stepfather for removing her from Scott's custody to "protect" her from the superhero life. After Scott's death, she grew estranged from her surviving family for a long time after becoming Stature. In the movie, Cassie lives with her mother and stepfather, with whom she maintains a much closer relationship even as she supports her father against her stepdad.
Guardians of the Galaxy Vol. 2: In the comics, Krugarr was Doctor Strange's student and eventual successor. In the MCU, the two never met each other, and Krugarr was an accomplished sorcerer long before Stephen Strange even knew that magic is real.
With Tony Stark/Iron Man. Here, Peter idolizes and is always seeking to gain his approval with Stark acting as a stern father figure and mentor to him. As Uncle Ben has already been killed off by the time Spidey is introduced, Iron Man is functionally Peter’s only father figure in this setting. In the comics, their relationship is considerably dicier and Spidey actually predates Iron Man (and the Avengers) as a superhero there. If anything, he's historically been closer a friend to Captain America.
With the Avengers themselves. He became a superhero with dreams of becoming an Avenger, and is considered a close ally of theirs right until becoming a member. In the comics, Peter didn't join the Avengers until the '90s, and even then, only as a reserve member (where he took part in stories that featured everyone). Historically, Spidey has been much closer to the Fantastic Four as an Honorary True Companion, and has actually served on the FF before (notably during the Future Foundation era) with Reed being his father figure and Johnny being Heterosexual Life-Partners with him, and everyone regarding him as their closest family outside each other. Rights issues at the time with Fox (before the merger) meant that the Fantastic Four had to be swapped for the Avengers.
He had a much more sour relationship with Ned Leeds in the comics, even without the Brainwashed and Crazy. Here, Peter is best friends with the guy.
Betty Brant never dates Peter Parker here, and instead is a Shipper on Deck of Liz's relationship with him, and later, his relationship with MJ.
Liz Allan has different relationships with Peter, Flash, and Betty in Spider-Man: Homecoming.
In the comics, she was Flash Thompson's high school girlfriend and long-time friend who initially joined him in picking on Peter, before she warmed up to and gained a crush on the latter. In the film, Liz and Flash don't have any relationship outside of being academic decathlon teammates and she isn't too fond of him (like most of her classmates), preferring Peter from the start.
She and Betty shared a mutual jealousy in the comics for most of their history due to competing for Peter's affections, while in Homecoming, they are depicted as being fairly close friends.
Peter Parker's most famous job is as a photographer working for J. Jonah Jameson at The Daily Bugle. This doesn't happen in the MCU, because Jameson is given Spider-Man's Secret Identity and outs him to the world in Spider-Man: Far From Home before ever meeting, knowing, and employing Peter Parker in any capacity. In the comics, Jameson got to know Peter as an employee and a one-sided surrogate son-nephew, even becoming an ally of Spider-Man in Ultimate Spider-Man. In Chip Zdarsky's run on Spectacular Spider-Man, he actually became Peter's friend and confidant.
Matt and his mentor Stick have a more antagonistic relationship than in the comics. It's not helped by the fact that the Netflix version of Stick is an Unscrupulous Hero, Knight in Sour Armor, Grumpy Old Man. While they did have friction in the comics, it was nowhere near as bad as Stick was more of a Pragmatic Hero, with noble goals, and only saw killing as a last resort. He was also a bit more compassionate towards Matt and had more Jerk with a Heart of Gold moments than the Netflix version.
While Matt and Frank Castle are still at odds philosophically, they have a much better relationship than in the comic books. In the comics, Matt and Frank pretty much hate each other and feel at best only grudging admiration for one another's abilities and determination. Here Matt and Frank have a relationship that is more like Friendly Enemies and the two of them come to understand if not condone the other's point of view.
Luke Cage: In the comics, Luke Cage and Iron Fist are Heterosexual Life-Partners who've headlined their own series together and often serve as Those Two Guys of the Marvel Universe. Here, they don't even meet until The Defenders (2017) and they have a fight. Even afterward, they still aren't portrayed the way they are in the comics. This does seem to be changing with Luke Cage Season 2, where the two bond.
Loki (2021): The TVA from the comics holds a great deal of animosity towards the Avengers, due to them frequently causing time paradoxes that they have to fix. Here, the TVA is shown to be dismissive at worst towards the Avengers when Loki accuses them of breaking timelines, even stating that they actually expected the Avengers to travel back in time to steal the Infinity Stones from main Loki's timeline, and it was Loki who messed things up by taking advantage of an opportunity that presented itself.
Hawkeye: In the comics, Jacques "Jack" Duquesne adopted and trained Clint Barton. In the MCU, they only meet long into Clint's career as Hawkeye, and even then their relationship is mostly as mutual acquaintances of Kate Bishop.
Oberst-Gruppenfuhrer Roeder visiting Schmidt is not to inspect on the progress of HYDRA's weapons programs, but to order Schmidt to report to Berlin for HYDRA's failure to stop Captain Carter, the SSR, and the Allies in a series of battles.
HYDRA is established as the Nazi Supernatural Sciences Division in Captain Carter's universe, rather than as the Nazi Deep Science Division.
