It's common for an anime to significantly change certain parts of the manga it's adapting for creative and practical reasons. This can affect the manga's characters in such a way that they either feel familiar yet different, or they become new people entirely. The best kinds of changes are those that impress viewers and improve the story.
Whether it was thanks to the animators undoing certain things from the manga or making up something entirely new, these manga characters worked a lot better in anime. Previously forgettable side characters now had the chance to be the stars of their own arcs or spin-offs, while protagonists became more compelling they ever were.
Warning: Spoilers ahead.
10 Makoto Itou
School Days
Since School Days was a visual novel, the player character Makoto Itou didn't really have a defined identity. At most, he was a decent blank slate who could lose himself to lust depending on the player's choices. The manga retained Makoto's unremarkable personality, while the anime turned him into a despicable philanderer.
In the infamous School Days anime, Makoto was a deceitful cheater who emotionally manipulated Kotonoha and Sekai into doing his bidding. Although the anime turned Makoto into a creep, it at least gave him a concrete personality. This added depth made it easier for viewers to understand who Makoto was, then hate him.
9 Tier Harribel
Bleach
Bleach's Espada were the most badass and complex shonen villains of their time, but the same can't be said for the poorly-written Tier Harribel. The third-ranked Espada had the barest development and backstory. She even suffered the Espada's most undignified exit when Sosuke Aizen "killed" her before she unleashed her full power.
The anime did Harribel justice by expanding on the origins of her protectiveness over the Tres Bestias, and loyalty to Aizen. Even the anime's comedic take showed more of Harribel's depths than the manga ever did. Bleach's finale, Thousand-Year Blood War, has the chance to continue Harribel's improved characterization.
8 Shin
Fist of the North Star
Shin was Fist of the North Star's first major villain, and the honorable Kenshiro's nemesis. Shin betrayed Kenshiro (his childhood friend), left him for dead, kidnapped Kenshiro's lover Yuria, then founded a brutal empire. Despite these, Shin died before the manga's first volume ended. He was quickly forgotten about soon after.
Shin's basics stayed the same in the anime, but his villainy was expanded. Besides having more scenes and ruling over some of the manga's notable villains, Shin even survived a fight against Raoh and lived long enough to die by Kenshiro's fists. Shin's potential as Kenshiro's rival was only realized in Fist of the North Star's anime and movies.
7 Inspector Koichi Zenigata
Lupin III: The Woman Called Fujiko Mine
Ever since Lupin III began in 1967, Inspector Zenigata was Lupin's bumbling rival. Zenigata was Lupin's comical foil. He worked with and for the law, and he was plagued by perpetual misfortunes as opposed to Lupin's endless luck. This changed in the prequel The Woman Called Fujiko Mine, where he became a legitimate threat.
This Zenigata was a ruthlessly efficient lawman. He wasn't afraid to get his hands dirty if it meant enforcing the law, and he only saw Lupin as a nuisance to arrest. Even though Zenigata's tough streak would be canonically undone when the classic Lupin III's story began, this was one of the rare times when he was a worthy opponent, not a punchline.
6 Seras Victoria
Hellsing (2001)
One of the few things that Hellsing's first anime got right was developing the characters. Seras Victoria was a good example of this. Seras was a naive cop who became a vampire almost on par with Alucard. She quickly got over her vampirification in the manga and Hellsing Ultimate, but she took more time to do so in 2001.
Seras spent the first half of the questionable Hellsingreeling with her second life. She also showed more of her expressive and optimistic side here. These gave Seras the kind of humanity that she and most of Hellsing's cast lacked in the manga and OVAs. In the latter two, Seras was a badass action hero in a cast overflowing with them.
5 Lust
Fullmetal Alchemist (2003)
Lust may have been one of Father's Homunculi, but she was one of the first Fullmetal Alchemist villains to die. In both the manga and Brotherhood, Lust was a one-dimensional monster who was killed before the final battle's preparations even began. Conversely, Lust was a complex and tragic antagonist in the 2003 anime.
Since the 2003 anime told its own story, its Lust was a remorseful villain who wanted to become human so that she could die. She even helped the Alphonse brothers in the finale, and died redeeming herself. Lust was one of the reasons why some fans of the shonen classic Fullmetal Alchemist preferred the 2003 version over Brotherhood.
4 Saten Ruiko & Uiharu Kazari
A Certain Scientific Railgun
Saten and Uiharu weren't just Misaka's and Kuroko's classmates; they were their close friends and A Certain Scientific Railgun's secondary protagonists. They backed up Misaka and Kuroko during major fights, and they partied together as friends whenever they could. They even got some anime-only arcs made exclusively for them.
None of these happened in the light novels and manga, where they were little more than background characters. Saten only appeared for one major arc, and Uiharu was a random Judgment member. Saten and Uiharu becoming full-fledged characters in A Certain Scientific Railgun gave the story more humanity and charm.
3 Goku
Dragon Ball Z (English dub)
To English-speaking Dragon Ball Z fans, Goku was the ideal father and role model. Although he seemed too carefree for his own good, Goku was still a loving dad to his family and Earth's selfless champion. This depiction only applied to the overlong Dragon Ball Z's localization, since Goku was more immature in the manga and Japanese dub.
Goku was basically an overgrown child in the manga. He was so naive that he could barely navigate his way through society. He also fought intergalactic villains because it was fun, not because it was the right thing to do. Goku's dubbed version clashed with his original incarnation, but he was infinitely more likable and relatable.
2 Yami Yugi
Yu-Gi-Oh! Duel Monsters
Before Yu-Gi-Oh! Duel Monsters became the premier tie-in anime, it was a horror manga. Yugi Muto was possessed by the vindictive spirit of Yami Yugi. Yami exacted justice on cruel people by subjecting them to dark games. Conversely, Yami in the anime was a heroic gamer. At worst, he had some demons to overcome.
The friendlier Yami was a result of the manga being rebooted to turn Yu-Gi-Oh! into the blockbuster it is today. Besides focusing on the Duel Monsters game, the reboot toned Yami's edge down. It also made him a clueless fish-out-of-water in the modern world. This change made Yami more lovable and iconic than his original self ever was.
1 Major Motoko Kusanagi
Ghost in the Shell
When it comes to manga characters being radically changed for the anime, nothing comes close to Ghost in the Shell. In the manga, Major Motoko Kusagnagi was a spunky and sexy action hero. She also spent her free time providing fanservice for other characters and readers. The Major's persona was the norm for cyberpunk manga of the time.
But when Ghost in the Shell was adapted into a movie, The Major was transformed into an emotionless cyborg who yearned for humanity. She was a Section 9 special agent who often questioned her profession's morals, or lack thereof. This version of The Major was so influential and well-received that she supplanted her original, sillier self.