The power of friendship knows no bounds in the wonderful world of anime. It has united communities, given hope to the hopeless, and given main characters some of the strongest plot armor and inexplicable power-ups ever witnessed in fiction. Best of all, anime and its wonderful friendship tropes have brought about some of the best redemption arcs and changes of heart that have ever been written.
It's almost novel at this point to see an anime villains change sides after either seeing the error of their ways or becoming entranced with the charisma and optimism of their protagonist counterparts. Yet, it never gets old. Time and time again, it's inspiring to see otherwise despicable or tortured characters change and, in so many words, become heroes.
Vegeta (Dragon Ball)
While Vegeta has had a lot of Saiyan transformations in Dragon Ball, his transformation from evil to good has definitely got to be his best. While it would've been easy, especially by Dragon Ball standards, to have Goku simply overcome Vegeta to show the power of humanity, Akira Toriyama managed to surprise and delight everyone by having that humanity or, at least, Goku's surprising resilience challenge Vegeta and turn him into rival and even a comrade.
This has lead to plenty of great fights showing the Prince of the Saiyans fight for himself and Earth and, better yet, some surprisingly heartwarming moments with his Earthling family.
Crona (Soul Eater)
Based on pure aesthetics, it's hard for many on a first watch to even see Crona as a villain. The kid is always visibly shaken and is always reluctant to fight. However, due to the unfortunate circumstance of being Medusa's child, Crona has had to put up with a life of experimentation and torment in an effort for Medusa to literally turn him into a monster.
That tragic background and Crona's own, natural cinnamon bun energy made his turn to good all the more cathartic and necessary. Crona not only found friends that he could rely on but also started fighting for himself. With the added benefit of turning Ragnarok into a cute, cartoony mascot, Crona's character arc was fun.
Nico Robin (One Piece)
When Nico Robin first debuted, she was the vice president of a conspiratorial, bounty-hunting agency that tried to assassinate the Straw Hats and Princess Vivi (presumably). She quickly abandons Crocodile for Luffy's crew after the sandman tries to kill her.
While it was easy for many, especially Roronoa Zoro, to think that she was just using the Straw Hats, her time with them eventually developed something new and special. Her loyalty and the Straw Hats' feelings towards her were tested in grand fashion during the Enies Lobby Arc, where Nico Robin officially and unambiguously changes for the good in easily her most iconic scene.
Gajeel Redfox (Fairy Tail)
Gajeel Redfox could've easily been just another villain of the week for Fairy Tail. The series and battle shonen as a whole are never in short supply of buff, angry guys just trying to wreak a lot of havoc. Even when he eventually did join the team, a lot of people were skeptical of his true intentions given how reluctant and gruff he still appeared.
However, those perceptions quickly changed when Gajeel showed how willing he was to fight for good and after the series excitingly revealed that he was Fairy Tail's own secret agent against Ivan Dreyar. To make matters better (and spicier), Gajeel even begins a romance story with Levy McGarden, one of the Fairy Tail wizards that he ambushed during his debut.
Gaara (Naruto)
Naruto is one of the best shonen franchises and it specializes in the art of friendship speeches winning over bad guys. There are plenty of Naruto villains throughout the series who either redeemed themselves in some fashion or joined Naruto's side entirely. This ex-rogues gallery includes fan favorites such as Zabuza, Nagato, and, despite being an evil scientist, Orochimaru.
However, the most memorable of any moral turn in this series has to be Gaara. Gaara debuted as the anime version of the child from Omen. He was cold, powerful, and so bloodthirsty that he could strike at any moment. After Naruto manages to headbutt some character motivation into him during their fight, Gaara quickly becomes a friend and even a noble leader for the Village Hidden in the Sand.
Takasugi Shinsuke (Gintama)
Contrasting Gintama's comedy world, Takasugi Shinsuke was the lone edgy character who countered Sakata Gintoki as the series' main villain. Though he was used sparingly, Takasugi's every appearance appeared impactful and threatening, as he teased and weaved threads for a greater, anarchic uprising against those in power.
His eventual turn to the good side was a great one for character who had been seething in his own misery for so long.
Scar (Fullmetal Alchemist: Brotherhood)
Scar is a shonen villain who, in the messy politics of Fullmetal Alchemist's world, could be seen as more of an anti-hero...if he didn't murder a lot of arguably innocent people. The byproduct of a bloody, ideological war and the victim of a grander conspiracy started by Father, Scar's entire character centered around hate and violence.
Even when he sided with Ed and Alphonse, his decision was more pragmatic in an effort to change his world's overlying corruption, though it would inevitably lead to redemption and peace in one of Fullmetal Alchemist's best character arcs.
Hiei (Yu Yu Hakusho)
Before there was Sasuke Uchiha, Hiei was the edgy, bad boy of every shonen fan's dreams. He debuted as one of Yusuke Urameshi's most dangerous rivals during his Spirit Detective days. After he was caught, Hiei reluctantly joined Yusuke's side as part of his parole, a la Suicide Squad rules.
While it was questionable to put a murderous criminal like Hiei into this position, he quickly became one of the Spirit Detective's most vital allies; and after meeting and reconnecting with his long, lost sister, he eventually grew to enjoy their company, whether he chooses to admit so or not.
Teruki Hanazawa (Mob Pyscho 100)
Teruki Hanazawa is one of the most recent examples of an unlikeable, arrogant villain becoming a heroic rival. In Teruki's case, he is one of the first people that Mob meets who also has psychic powers.
However, instead of using them for good (or, at least, Reigen's business), Teruki uses his powers to rule over Black Vinegar Middle School as well as threaten anyone who opposes him. Mob quickly comes to the scene to defend his friends and humble Teruki. Even though Teruki still views Mob as a rival, he significantly calms down as a character and has consistently been an ally and protector to Mob and his friends.
13 Court Guard Squads (Bleach)
In any other anime series, the overlying organization defining the series' main thematic element (e.g. ninjas, Saiyans, pirates, samurai, etc.) would include its heroes. Bleach, however, decided to hype up its main crew by debuting them as challengers instead of allies.
The entirety of the 13 Court Guard Squads were villains in their own right (especially the creepy scientist one, Mayuri) who took Rukia for execution. This led to Ichigo's iconic invasion against the Soul Reapers that had him and his friends take on several of the Soul Society's powerful captains. When the smoke cleared and a new, mainline villain was formed, Ichigo eventually joined sides with the 13 Court Guard Squads and relied on them as allies for countless battles in the series' future.