Two of the biggest changes an anime could do to its source material is to expand an existing side-character's role or create an entirely new one. If done right, fans would love these newcomers even more than their canon counterparts.
This is the case for these previously non-canon characters, who either never existed in the light novels or manga their anime was based on or were almost insignificant before they were given an upgrade in canonical follow-up materials.
WARNING: Spoilers ahead.
10 Puella Magi Madoka Magica: Rebellion — Homulilly Was First Seen In A Video Game
Though it's a given that every magical girl is doomed to either die or become a Witch, not all of the main magical girls' Witch forms were shown over the course of the anime's episodes. The closest glimpse of Homura, Kyoko, and Mami's cursed forms fans got was in the video game Puella Magi Magica Madoka Portable, where they could be fought as bosses.
Despite not appearing anywhere else, these Witches were popular enough that one was brought into canon through the movie Rebellion, namely Homulilly. Not only that, but it was revealed that the movie's world and events were actually born from Homulilly's labyrinth, where Homura was just a few steps away from fully becoming her Witch.
9 Mobile Suit Gundam — The Black Tri-Stars Became More Prominent In The Origin
Given its vast scope, Mobile Suit Gundam unsurprisingly has tons of cool soldiers who are doomed to die in combat. One such group was the Black Tri-Stars, a trio of ace pilots from the Principality of Zeon. Despite only appearing in two episodes, the Tri-Stars quickly became fan-favorites thanks to their iconic MS-09B Dom mobile suits and attack patterns.
The trio got their due in Mobile Suit Gundam: The Origin, the anime's official manga that also served as a prequel to the One Year War. Here, the Black Tri-Stars' military service was fleshed out and given more light. Not only were the three revealed to be some of Zeon's first mobile suit pilots, but their fierce rivalry with Char Aznable was also introduced.
8 Bleach — Katen & Kyokotsu Came Back In Time For The Final War
Of Bleach's many filler arcs, "Zanpakuto: The Alternate Tale Arc" stands out the most because mangaka Tite Kubo was actually involved in its making. Specifically, Kubo created the Zanpakuto souls that rebelled against their Shinigami. Though the spirits were immensely popular, they didn't show up in the manga before and after the filler episodes.
This changed during the manga finale, "The Thousand-Year Blood War," when Captain Kyoraku's two-pronged zanpakuto, Katen Kyokotsu, manifested to help him survive fighting Lille Barro. Though Katen and Kyokotsu were the only anime-only zanpakuto to show up, their presence alone officially confirmed their fellow spirits as canon.
7 A Certain Scientific Railgun — Saten Ruiko & Uiharu Kazari Got Bigger Roles In The Manga
While Saten and Uiharu exist in A Certain Magical Index's light novels and manga, they were just background characters. Saten was integral to the Level Upper incident and nothing more, while Uiharu was one of Judgment's more prominent named members. This changed in the Railgun anime, where they were promoted to main character status.
As Mikoto Misaka's friends and comic relief duo, Saten and Uihari got more development and screen time than before. The two became so popular that their roles in the anime's ongoing source materials were beefed up significantly. Just like in the anime, Saten and Uiharu were now two of Misaka's closest friends and romantic supporters.
6 Love, Chuunibyo & Other Delusions — Sanae Dekomori, Kumin Tsuyuri & Touka Takanashi Joined The Light Novels
One of the bigger changes that the anime adaptation of Love, Chuunibyo, And Other Delusions did to the original light novels was to give Rikka and Yuuta more friends. The most prominent additions were their anime-original schoolmates Dekomori (left) and Tsuyuri (center), who were a fellow chuunibyo and a narcoleptic upperclassman respectively.
The most important addition, however, was Rikka's domineering older sister Touka (right). Not only did Touka ground Rikka out of a sense of tough love, but she provided the anime with its most personal conflicts. All three quickly became fan-favorites, and they were formally introduced into the light novels sometime after their animated debuts.
5 Dragon Ball Super — Broly Became One Of The Franchise's Most Popular Villains
As iconic as they are, Dragon Ball's movie-only villains like Cooler and Lord Slug were considered to be non-canon. One of the few cinematic antagonists to escape this non-canon status was Broly, the legendary Super Saiyan and nigh-unstoppable galactic warrior. Broly officially entered the canon in his movie, Dragon Ball Super: Broly, 25 years after his debut.
Broly first appeared in the movie, Dragon Ball Z: Broly - The Legendary Super Saiyan, where his creators thought he'd just be another cool new villain. Instead, Broly became one of Son Goku's most famous foes, so much so that he was given the most number of video game appearances and transformations among his fellow Dragon Ball villains.
4 Pokémon: The Series — The Team Rocket Trio Joined The Games
The Pokémon anime is notable for being a really loose adaptation of the games it was based on, so much so that it stars anime-original protagonists Ash Ketchum and his Pikachu. That being said, they were arguably overshadowed by their anime-only rivals: the Team Rocket Trio, which was comprised of Jesse, James, and Meowth.
The trio may have been conceived as Ash's bumbling antagonists, but fans loved them so much that they were upgraded into anime anti-heroes and added to the canon games. The three became recurring bosses in titles like Pokémon Yellow, its remake Let's Go, and the mobile game Pokémon Masters, all of which were made long after their debuts.
3 Rebuild Of Evangelion — Mari Illustrious Makinami's Fan Theories Became Canon
For the most part, the Rebuild Of Evangelion movies (which were sequels to Neon Genesis Evangelion) brought back the original cast. The one exception was Mari, the new Fourth Child and the pilot of Eva Units 05 and 08. Despite her lack of a backstory and prior connection to the cast, Mari proved popular enough to inspire fan theories.
Some guessed that Mari was Yui Ikari's college friend from the manga, though this would mean she'd be decades older than Shinji despite her teenaged appearance. This fan theory was canonized in the series finale Thrice Upon A Time, where Mari was revealed to indeed be a part of Yui and Gendo's friend group who somehow de-aged herself to help Shinji.
2 Yu-Gi-Oh! Duel Monsters — Many Of The Anime-Only Cards Became Official
It's obvious that Yu-Gi-Oh! Duel Monsters was made in part to advertise pre-existing cards sold in hobby stores or used in tournaments, but a good number of them were made specifically for the anime. Besides giving Yami Yugi and his fellow duelists unique Monsters to summon or Spells/Traps to cast, some of these cards were practically characters of their own.
The more popular certain anime-only Monster cards were, the more likely they were to be made official and printed for sale. Some notable examples include Rafael's Guardian Eatos, Yami's Kuribandit, Zigfried's assortment of Valkyries, and all three of the legendary Egyptian God Cards, which were passed between duelists.
1 My Hero Academia — Bubble Girl Was A Fan's Original Character
One thing anime fans really love is making OCs (original characters) and imagining them interacting with their favorite anime's world and characters. Artist Abara Chitta was no different, as seen in her original pro-hero, Kaoruko Awata, aka Bubble Girl. What makes Bubble Girl special is that she's officially part of My Hero Academia's canon.
In 2017, Chitta entered Bubble Girl in a My Hero Academia character designing contest hosted on the artists' site Pixiv. Bubble Girl was so well-received by both fans on Pixiv and mangaka Kohei Horikoshi that not only was she added into the manga's canon as one of Sir Nighteye's sidekicks, but she made her animated debut in the third season.