In his earliest appearances, while Batman wasn't any more particularly kill-happy than any other heroes, it's kind of shocking to see the archetypal Technical Pacifist clearly killing off criminals, often using guns. Case in point, a line from a showdown with his first nemesis:
Batman:[as he watches the man in question burn to death] Death... to Doctor Death!
He also displayed a fondness for puns and cracked jokes during fights, not unlike what Spider-Man would do later. IE, "Have a seat", while smacking villains with a chair, or, while beating the Joker "You may be the JOKER, but I am the KING OF CLUBS!" or "You played your last hand!". Now, he's oh so very much The Stoic.
And, on more than one occasion, he referred to himself as "Poppa", in the third person, as in "Quiet, or Poppa spank!" or "Right into Poppa's arms!".
The first Catwoman story depicts Batman deliberately allowing her to escape purely because he thinks she's hot, and then joking about it with Robin. Ever since then, he's been nothing but angsty about his obsession with her.
The Joker was originally a sneering, humorless criminal mastermind instead of the cackling lunatic we know today. He was named after the Joker in playing cards, seeing as how he never told a single joke, and never laughed in his initial appearance (though he did smile, rather terrifyingly). The Man Who Laughs later rewrote the Joker's first story with the current portrayal of the Joker.
Catwoman was, like Batman, shown to kill people in her earlier appearances, when she would later become established as a Classy Cat-Burglar whose morally-ambiguous or outright heroic nature rested on her scrupulous refusal to kill the innocent. (But not other villains, as Black Mask found out. Twice.)
Selina's whole backstory has changed since she was first created as well. Especially now that she and Batman are more often than not an Official Couple after she learned his secret in Batman: Hush (that ran from 2002-2003). She used to be a rich socialite as well who robbed because she was bored. Now she's a poor street orphan who robbed to take care of her sister as a way to play up the Uptown Girl angle in their relationship.
Robin was also shown to use brutal force on crooks in his debut, hitting them with rocks from a sling and shoving them off scaffolding to their deaths. This first origin story also depicted him as eight years old while doing all these feats (compared to later retcons that would establish him as either ten or twelve).
Also Alfred is remembered as the Wayne family's loyal butler, however he was originally written as a Clueless Detective who wasn't quite up to par with the World's Greatest Detective, Batman. Rather than swear revenge, he ended up serving in the Wayne Manor and the Batcave. Nowdays Alfred is just written as The Jeeves.
Alfred was also originally overweight and clean shaven, but adopted his iconic thin, mustached appearance after a stay at a health spa.
Alfred also didn't become a major character until Denny O'Neil's run in the 70s. Bruce had previously been raised by an uncle.
Renee Montoya's sexuality is a defining character trait for her under Greg Rucka, but when she was originally created for Batman: The Animated Series the plan (according to background info in the series bible) was that she was intended to be driven in her own fight on crime by the memory of her dead husband. It was not until Gotham Central that she was outed as a lesbian, which would become an integral part of an award-winning arc of that series and several followup comics. The same source also says that she would butt heads with Batman over his methods, whereas in almost all her appearances Renee admires him and understands why he acts outside the law.
Batman's suspicion and hostility towards Superman is a relatively new thing. Prior to the '80s, Bruce and Clark got along just fine. By 1985, however, the two were established as polar opposites who clashed over varying differences.
Original Superman was an outlaw hero not above dispensing rough justice in his earliest appearances. It is a little shocking for modern readers to see the character who would later become 'the Big Blue Boyscout' seize a torturer and fling him to his death.
DC later retconned these early out-of-character moments as being the Superman and Batman from Earth-2, so they're technically separate characters from the "main" Superman and Batman. Shows that they were aware just how much characterization had marched on in all those years.
John Byrne acknowledged this in Superman & Batman: Generations as part of the characters' evolution over time. In 1939, Batman takes one of the Ultra-Humanite's goons on top of a giant planet model to interrogate him and lets him fall when the mook doesn't give any useful information. When he's caught by Superman, Bat-Man remarks "If I'd known you were there to catch him, I wouldn't have let him fall", and Superman responds "If I'd known you'd let him fall, I wouldn't have caught him." They resume the interrogation and, when the mook still refuses to talk, Superman makes as if to throw him off the sphere once more, at which point he finally sings.
The New 52 has also characterized young Superman similarly as his early incarnation.
It's kind of a shock to see how Guy Gardner was originally a pretty mellow character before (in a convoluted series of events) he received brain damage that manifested itself in the form of the arrogant, violent, unstable, and often childish personality that the character is best known for. DC has since tried to sweep this under the rug by retconning the brain damage thing altogether. Some flashbacks imply that Guy has always been, as he puts it, "the crazy one." Likewise, those who are most familiar with the mostly-stoic Marine in the DCAU version of John Stewart, which has since been incorporated into the comics, may be surprised at how, well, funky he is in his early comics appearances.
In The Silver Age of Comic Books, Sinestro was very generic and banal in his evil; his conquest of his planet (seen in flashback) involved him sitting on a throne, demanding tribute in the form of money and jewels in exchange for his services as protector and killing anyone who called out on what a selfish lout he was. It wasn't until 1991's Emerald Dawn II, that Sinestro's past was completely reworked to make him a Adolf Hitler-esque control-freak who conquered his homeworld "to protect it" as the driving force behind the evil he did.
Deathstroke was far more of a dirtbag in his earliest appearances, in sharp contrast to his eventual Anti-Villain status. One of his earliest appearances had him kidnapping Cyborg's love interest in order to lure the Teen Titans into a trap, something that would seem at odds with his Never Hurt an Innocent guidelines that would be established in later issues, and recreate the Doom Patrol's way of dying - a traumatic event for Changeling - For the Evulz. This can be explained by the creative differences between George Perez, who saw Slade as a ruthless villain whose "code" was only a personal justification, and Marv Wolfman, who did not see him as a villain but as an honorable victim of circumstance. Thus when Perez left the title, Deathstroke's more noble characterization took hold. However, Christopher Priest eventually restored Deathstroke to his original characterization in Deathstroke (Rebirth).
In the original comics, Plastic Man was no less serious than any other superhero at the time - he was the Only Sane Man of his world, which was filled with wacky, cartoony characters. In fact, even his powers were amusing, which made for an enormous contrast with his early, comically serious personality. Over the years, Plas received a much more light-hearted, jokester disposition, being mostly comic relief and Fun Personified in his modern portrayals, such as the one in Batman: The Brave and the Bold.
When she was first introduced in Legion of Super-Heroes, Monstress was basically a female Ben Grimm, in both speech patterns and attitude. Once she joined the team, the writers decided it was more interesting if this hulking monster was an upbeat fashionista who called everyone "dear" or "sweetie".
Green Arrow was originally and for a long time after his introduction mild mannered and apolitical - essentially a better adjusted equivalent to Batman, at a time when even Batman was more prone to smiling than his current incarnation. His left wing politics and spikier personality generally only became part of his character in the late Sixties, nearly three decades after the character's introduction.
Terra as originally written in The Judas Contract has become this over the years. In the original Teen Titans comics from the 1980s she was canonically "psychopathic" and "evil". She had no ulterior motive for being The Mole or hating the Titans besides just being cruel and hating "goody-two-shoes" people. Word of God is that Terra's Age-Gap Romance with Deathstroke was there to make her seem even more gross. Terra's original interpretation however has caused a lot of Values Dissonance, especially with people criticizing how the Slut-Shaming factor to her relationship with Deathstroke and noting how Terra is vilified while Deathstroke often gets treated far more positively. As a result, since the 2000s DC has made an emphasis to portray her as a less one-dimensional character, being a troubled youth who fell in love with an older man who ended up emotionally manipulating (or even drugging in some comics) her. She's still morally ambiguous but she's evil For the Evulz.
Amanda Waller was originally a tough, intimidating woman who also had a strong moral code. Indeed, much of her meanness was just an act, and she was willing to go against orders for the sake of her Suicide Squad. When she believed that the U.S. government had been corrupted, she took the Squad and went rogue. These days, however, most readers know her as a sociopathic manipulator who believes in My Country, Right or Wrong and throws away the lives of the Squad at the drop of a hat.
Spider-Man: From its origins in 1962 to about 1994, it was known for having dynamic real-time characterization where characters grew and changed, even after it adopted a sliding time scale. Death was death and so on. The Clone Saga ended its realism, and One More Day exchanged the dynamic characters with Static Character. Likewise most of the story is told from Peter's POV and we rarely see the story from the viewpoint of other characters, so their characterization marched on at a different pace:
Pre-OMD, you had Spider-Man who started off as hotheaded and ready to fight for little reason. As early as Amazing Spider-Man #1, Spidey breaks into the Fantastic Four's house to fight them in order to prove his worth as a potential member. He mellowed out once he became an adult so a lot of this could be chalked up to him being a teenage boy who just got super powers.
Gwen Stacy was easily the most inconsistently written character in the classic period. When co-creator Steve Ditko was around, he consistently wrote her as a stuck-up college-aged Alpha Bitch and beauty queen who got in because of her class and looks. Then after he stepped down, and John Romita wanting a more regular social circle and a change of scenery, had her rewritten into a more virtuous girl. Then her father was introduced and she became a weepy Daddy's Girl who largely followed the men in her life. Most notably she was the Veronica and then the Betty in the ongoing Love Triangle with Peter and MJ, before winding up Spider-Man's very sweet girlfriend, and then, her father died which she blamed on Spider-Man, which did a number on her relationship with Peter who was convinced that she wouldn't accept his double life at all. Then there came The Night Gwen Stacy Died, where posthumously she became a Satellite Character for Mary Jane (right from her death issue, where MJ is easily the most important female character), and MJ often revealed her more vulnerable side whenever Gwen came, feeling upset about her poor background compared to hers and guilty about taking her place in Peter's life. Her later portrayal (Peter's one true love who was Too Good for This Sinful Earth) didn't exist until after her death.
Mary Jane Watson was set up by Aunt May as a Blind Date for her wallflower love-shy nephew (as she saw it) even if Peter felt he was actually doing a good job getting past that (he wasn't but he did have a relationship with Betty Brant). Aunt May kept hyping MJ as an ideal match for Peter and readers, after being told contextually that she was indeed very beautiful, were in suspense for her introduction. And when revealed in #42, as a charismatic Audience Surrogate (a 60s party girl who thought Spider-Man was cool and so was Peter), despite her characterization by Lee and Romita as a flaky party-girl, was seen by fans to be the more interesting character. One of those fans, Gerry Conway, wrote the death of Gwen Stacy specifically to develop her character and revealed a more courageous, compassionate, and loyal side than previously expected. Later writers, Marv Wolfman, Roger Stern, Tom Defalco revealed a much more complex character and made her one of the most important supporting characters not just in Spider-Man but Marvel Comics as a whole, and finally Peter's wife, and Post-OMD still his best known and most popular love interest.
Aunt May is another example. In the beginning, she was a sweet, extremely old and extremely frail old lady but, readers also felt that she was somewhat senile and detached from reality. That she allowed herself to be charmed by Doctor Octopus and saw Mary Jane Watson as a suitable partner and future wife for Peter (before MJ's characterization marched on) was portrayed as evidence for this by writers. Then in the late 1970s she became more involved in the real world, e. g. joining the Gray Panthers, a bypass operation removed her recurrent health problems, and by all appearances she actually became younger. During Roger Stern's run her reasons for continuing matchmaking also was revealed as much more canny than previously imagined; she commented to Peter that both he and MJ "had lost so much" which stunned Peter when he realized that in all the time they dated, he never asked MJ about her life and later he learned that MJ really was someone he had more in common and that his Aunt was right about her all along. Writers also implied that Aunt May knew Peter's secret, which was confirmed in the wonderfully written (but later retconned) issue #400. After the Clone Saga, May discovered the secret a second time in JMS' Spider-Man, has a sane conversation with him about it, and is totally able to deal with it, making her the coolest old lady on Earth. Later episodes have her helping with the secret identity in ways that make you wonder how he ever got along when she didn't know. Then, it was all retconned a second time.
In the Post-OMD era, with Static Character for all, in essence characterizations are composite from different periods in history since the undoing of the marriage and the concurrent maturity and growth that comes from characters making that commitment, meant that Peter in Post-OMD is an unlucky but optimistic Butt-Monkey and Manchild, Aunt May remarries, grows younger, and lives apart from Peter but still remains his only family. Mary Jane is Peter's on-off girlfriend, sometimes bitter, sometimes loving and friendly, other times distant, and other times flaky like always.
In the first two or so issues of, Beast essentially sexually harassed Jean Grey and was a big dumb oaf, but as early as the third issue, he became a self-described bookworm who used a much more impressive vocabulary and was far kinder to people around him, and it was treated as if he had always been so. Interestingly, in a much later arc, the Beast, now blue and furry (which is also this trope, as some don't know he wasn't always this way), has his appearance restored to his original human form, but at the cost of his intelligence dwindling as he uses his powers. He eventually talks much like his day-one self.
The original plan for the X-Factor comic (where Apocalypse was first introduced) was that the shadowy mastermind that X-Factor had been fighting against was planned to be minor Daredevil and Spider-Man villain the Owl. When a new writer came in, they decided to create a new villain.
Magneto himself is a good example of this. The Stan Lee/Jack Kirby version of the character was just a straightforward villain with essentially no character depth. Chris Claremont gave Magneto a sympathetic backstory and changed him into a Well-Intentioned Extremist, and these changes have more or less stuck over the intervening decades. Unfortunately, post-Claremont writers and editors re-cardboardified Magneto to some considerable extent.
Rogue got her start as a villainous henchwoman working for Mystique, taking on and nearly single-handedly defeating the entire team of The Avengers with a bravado that would be out of place on the troubled young girl she was soon developed into. This is particularly strange when you consider that nearly all of Rogue's earliest appearances - including that encounter with the Avengers - were written by Chris Claremont.
According to a tenacious urban legend, Wolverine's origin was intended to be that he was actually a wolverine mutated into human form. Len Wein himself, Wolverine's co-creator, has made it clear that this is just a myth. Old plans that were made are that his claws were originally intended to be built into his gloves, and Word of God states that he was planned to be about as strong as Spider-Man (i.e. at the low end of superhuman strength in the Marvel Universe, but still able to lift about 10 tons).
He also started out as a thug with a bad temper whose main contribution to fights was rushing in recklessly and getting swatted aside to prove that the villain couldn't be beaten without teamwork. During The Dark Phoenix Saga, he Took a Level in Badass and never looked back. It's also surprising (considering how important it is to his modern characterization) how long it is between his first appearance and the first explicit mention of his Healing Factor (he says that he heals fast a little earlier, but it's in more of a "don't fuss over me just because I got myself beaten" context than an "I have a superhuman ability to heal" one).
And how about Charles Xavier himself? In the first twenty or so issues, he used his powers a lot more, reading minds whenever he pleased, communicating telepathically even to people in the same room, harboring a secret crush on Jean Grey, and mind-wiping several of the X-Men's foes in order to get them out of the way for good. While some later writers would portray Xavier as a Jerkass, it never reached this level.
Also, Iceman. Nowadays, he is mostly known as a funny guy. In the beginning, he would often attack his teammates during mission briefings. While this was probably intended to show him as immature (he was two years younger than the rest), it just made him look like a Jerk Ass and an idiot.
In several of the early comics, the Green Hulk was slightly more intelligent and could talk better, barely even referring to himself in the 3rd person. Then eventually this paved way to the more popular dumb beast that always spouted "Puny human make Hulk angry! Hulk smash!" Though later retcons would establish that this behavior was still canon. The Hulk has multiple personalities, with some of them being quite intelligent while others are just mindless, screaming monsters.
Hannibal King, friend of Blade, was originally introduced in The Tomb of Dracula as a no-nonsense private eye who tended to stay calm, unobtrusive, and most professional. In Night Stalkers, he became very emo, angsty, and melodramatic. And in later appearances, he became a goofy, bumbling, comic relief punching bag. This third characterization may be a response to Blade: Trinity and Ryan Reynolds' characterization of King.
Before becoming Moonstone, Karla Sofen was introduced as a scantily-clad gun moll for the Captain America villain Doctor Faustus. There was also no hint of the cunning and manipulative personality she would later become known for.
Betty originally had Dumb Blonde and Clingy Jealous Girl tendencies. She was even something of delusional in her obsession with Archie and would make schemes akin to Wily Coyote. By the 70s she was written as the level-headed tomboy we associate her with.
Veronica was more aggressive and generally mean early on. She didn't get along wth Betty either. Over the decades she was toned down from an Alpha Bitch to a Lovable Alpha Bitch and is best friends with Betty.
Seems to have occurred with many Disney characters over decades of comic book appearances. Some examples:
Mickey Mouse. In "Mr. Slicker and the Egg Robbers" (1930), Mickey seems to be losing Minnie Mouse to Smug Snake Mr. Slicker (Montmorency Rodent). He even overhears a conversation where Minnie silently lets Slicker believe Mickey is just her "little brother". Mickey's reaction? He concludes "She doesn't care for me anymore - what is there to live for! Without Minnie, I might as well end it all". ... "I can't get Minnie off my mind! I just can't go on without her!" ... "Without Minnie, all my dreams have become nightmares" ... "Goodbye, Minnie! Goodbye, cruel word!". Only after four failed suicide attempts does he snap out of it. This is the same Mickey who would make an art of ditching Minnie to attend to his next mystery investigation/reporting/adventuring around the world. Later stories have made a running joke of the Mouse couple hardly spending any time together.
Phantom Blot. In "Mickey Outwits the Phantom Blot" (1939), the Blot is a rather scary opponent who easily overcomes Mickey on several occasions. With only one weakness. In his words "My cursed soft heart! I never could bear to SEE anything die! I'm just too tender for such things". So he sets elaborate death traps instead, leaving them to take care of Mickey. Mickey typically escapes with minimal injuries. The Blot remains highly efficient in most of his incarnations. But the soft heart is mostly forgotten about and he takes a more direct approach. For example in "The Hooded Eagle" (1994), the Blot has no problem attacking Mickey with an axe. After taking elaborate efforts to lure him to an isolated location of the Arctic, indicating he was planning about it for some time.
The Phantom Blot was in his early appearances portrayed as a master thief, and was actually unmasked at the end of his first appearances. Later he went from a thief into a full-blown super villain, and the fact he even had a face under the hood was forgotten (in one comic in particular Mickey meets the Blot's brother, who for some reason is also wearing a hood, and comments that he finds it hard to think of the Blot as a regular person with a family, instead of some kind of supernatural monster). He's started appearing unmasked from time to time, and has gone back to stealing things instead of trying to take over the world or whatever (although he still has access to various high-tech devices, usually stolen prototypes, that he uses to commit crimes).
Speaking of Disney, Scrooge McDuck. In his earlier appearances as a Donald Duck supporting character, Scrooge was mostly an antagonistic or even villainous character — there was the infamous robber baron flashback in "Voodoo Hoodoo", and in "The Magic Hourglass" Scrooge sics a band of hired thugs on Donald and threatens to leave his nephews to die of thirst in the desert if they won't give up the McGuffin he's after (though he isn't quite hard-hearted enough to go through with it). When Scrooge became the star of his own spinoff series, Barks had to make him more sympathetic and he quickly evolved into the proud and avaricious yet still honorable tightwad that we know today.
In his "Life and Times" seriesDon Rosa tried to explain the "Voodoo Hoodoo" incident as a huge mistake that Scrooge made as a young man in a moment of hubris and spent the rest of his life regretting; but this doesn't really fit with the Barks story where the present-day Scrooge is shown laughing at the memory.
His DuckTales incarnation takes a step even further, to the point of clashing with even the later comics version (and some early points of the show itself). While Scrooge remains somewhat curmudgeonly and closely tied to his money, he tames greatly, becoming more primarily a warm father figure for the triplets.
Gladstone Gander was only an arrogant dandy who served as The Rival for Donald in his first three appearances — him being Born Lucky on top of that wasn't established until his fourth appearance ("Race to the South Seas"), and even then this trait only existed intermittently for a while before becoming cemented as a permanent focus of his character.
In the original issues of Mandrake the Magician, Lothar was more or less Mandrake's negro manservant, never speaking, showing up whenever baggage needed to be carried or enemies needed to be beaten up, and vanishing between scenes when he wasn't needed. Once this became socially unacceptable, Lothar was rewritten as Mandrake's good friend, as well as a good deal smarter and proactive. On the other hand, Mandrake went from being a full-fledged wizard (kind of like a male Zatanna) to only being capable of using illusion magic.
Tintin, in his earlier adventures, tended to defeat his enemies by beating them up. His condescension to the natives and cruelty to animals in Tintin in the Congo are a jarring contrast to his humane attitude in the later adventures.
Originally, the Tintin stories reflected the attitudes of Herge's mentors, who encouraged him to write religious and political propaganda pieces (Tintin in the Land of the Soviets) and his work reflected European colonial attitudes circa 1930. A turnaround came when Herge befriended a Chinese student in Brussels who taught him about Chinese society, politics, and culture. As a result The Blue Lotus is critical of Western imperialism in China and criticizes westerners making racist comments about the Chinese.
In the early "Man of Iron" story in the Marvel UK Transformers Generation 1 comic, Optimus Prime orders that the Man of Iron and his navigator and ship be destroyed to prevent the Decepticons getting their hands on them. This is the same Optimus Prime who generally goes out of his way to save noncombatants.
Micronauts: Team leader Commander Arcturus Rann in the beginning had a more rakish personality somewhat like Han Solo. His speech included slang, mild swearing, and Solo-isms such as "Biotron, get your bolted butt back here!". Later, Rann's speech would become more mature and stilted as becoming of a legendary hero of the Microverse. On the other hand, Marionette, a princess of Homeworld, would start off speaking more formally, but later adopt hipster slang into her speech. Justified in her case as she is said to have spent some time on Earth away from the team.
Amy in Sonic the Comic is known as a badass Only Sane Man of the Freedom Fighters. Her earliest appearances, however, have her much like her game version would be known as. Amy is less proactive and teases Sonic a lot. This personality change was due to Executive Meddling, as it was thought female readers needed a strong female role model. She was mostly reversed in the final arc, the Adventure adaptation, which makes it seem odd when Amy is standing around instead of being in the action.
Jaime from the Spanish Superlópez series was originally created as López's work rival, who would report everything he did wrong to their boss in an attempt to get him fired and take his girlfriend. They bonded notably in book no. 10 and became friends from then on, with Jaime appearing now as a nice, caring man. His old personality can only be seen briefly in books 35 and 36.
In the first volume of The Demon Mages, the Gorgon actress Ari spoke without any pronounced accents. Outside of the comics, she's known for her Southern Belle-type accent. Only in the second half of the second volume does it begin to show.
Iznogoud: The Caliph's chamberlain introduced in Iznogoud's Birthday was initially not specifically aggressive to Iznogoud and starts acting antagonistic toward him as a reaction to his anger and obnoxiousness. When the same chamberlain is reintroduced in later comics, he is portrayed as being basically Iznogoud's Arch-Enemy who is trying to protect the Caliph against his overthrowing attempts. Being jailed at the end of Iznogoud's Birthday didn't help as well (although Iznogoud had good reasons to do that, to be fair).
The Splinter who is a loving, if flawed, father is nowhere in evidence in the first issue of Teenage Mutant Ninja Turtles, where he casually tells his sons that the reason he's been training them for years is so that they can attempt to kill a man they have never met to satisfy a vendetta they had never heard about before, a mission he intends for them to undertake with no actionable intelligence and no expectation of survival. While technically still canonical, future stories have had to gloss over these details, since they're completely at odds with the characters the turtles and Splinter would eventually become.