In any storyline, the protagonists and the antagonists have a definite role to play. While the villains may have their own hidden stories, their main purpose is to be a roadblock to the heroes and cause trouble. However, there are many instances when a third party arrives, and the heroes are forced to team up with the villains to defeat the greater evil.
There are many occurrences in various anime series where the villains have turned to the good side. However, not all villains take the righteous path permanently. Some of the villains just momentarily provide back-up to the protagonists so that they can temporarily achieve a common goal.
10 Team Rocket & Ash Vs Evil Malamar
Team Rocket is one of the consistent group of antagonists in the Pokémon anime series. While they do provide some comedic relief, Team Rocket is always after Ash and his friends to steal their Pokémon. However, there have been some instances when Team Rocket has also helped Ash escape from difficult situations.
When Ash and his friends along with Jessie of Team Rocket are captured by the Evil Malamar in episode 853, James and Meowth along with Inkay and Clemont devise a plan to help the others escape. By defeating the evil Malamar, Team Rocket stops them from changing the world's environment and also rescues Ash and his friends.
9 L & Light Yagami Vs Yotsuba
Light is the main antagonist of the Death Note series. Keeping Kira as his alias, Light is extremely smart and strategic, who initially uses the Death Note to get rid of the criminals. However, Light soon starts using the Death Note to kill anyone who comes in his way. To arrest Kira, L — a renowned detective — starts investigating and a battle of wits ensues between L and Light.
During the initial stages of the series, Light voluntarily joins L's team to keep a close eye on him, but he soon momentarily loses his memories of using the Death Note. During this time, Light assists L to catch Yotsuba, a group of businessmen who are using the Death Note to gain political power.
8 Lelouch & Suzaku Teaming Up For Zero Requiem
In Code Geass, Lelouch and Suzaku were formerly friends but turned into enemies due to their different ideals. While Lelouch was ready to use violence and terrorism to achieve his goals, Suzaku wished to attain the same without causing much harm. Their friendship turned sour, and they fought each other during the series.
However, Lelouch and Suzaku teamed up to launch the grand plan known as Zero Requiem. Through this plan, both Lelouch and Suzaku wanted to bring peace and atone for their sins. Since this was one of those moments when Lelouch and Suzaku agreed with each other, they both worked together to turn Zero Requiem into a reality.
7 Naruto & Sasuke Vs Madara Uchiha
While Sasuke is not a proper antagonist of the Naruto series, he has gone through a dark phase in his life. An eternal rival of the main protagonist, Naruto, Sasuke has always been at odd ends with Naruto and even walks down the dark path because of his dream to seek revenge. However, when Konoha is threatened by Madara Uchiha, he decides to help Naruto win the battle.
Since Madara is one of the strongest opponents faced by Naruto and Sasuke, the battle is a long and exhausting one. However, Naruto and Sasuke revive their teamwork and work together to defeat the former Uchiha leader.
6 Inuyashiki & Shishigami Vs Meteor
Inuyashiki tells the story of two individuals — middle-aged Ichiro Inuyashiki and high school student Hiro Shishigami — who get reconstructed into machine-like creatures with special abilities by the alien force. However, while Inuyashiki uses his power to help people, Shishigami turns into a cold-blooded serial killer.
Both Inuyashiki and Shishigami end up as enemies, however when a meteor is about to strike and destroy Earth, they join forces. Shishigami uses his self-destruct mechanism to destroy the meteor, but it does not get fully destroyed. To make sure the meteor does not strike the Earth, Inuyashiki also uses his self-destruct mechanism and kills himself to destroy the meteor.
5 Obito & Kakashi Vs Kaguya
Obito and Kakashi were teammates and friends when they were young ninjas, however, a misunderstanding and Rin's death soon turns their friendship sour. Growing up, Kakashi thought Obito was dead, but Obito was secretly working towards getting revenge in the future. War was declared by Madara and Obito against Konoha, and Kakashi fought against Obito.
Later, Obito realizes his mistakes, and he helps Kakashi fight against Kaguya Otsutsuki, Obito finds himself in between life and the afterlife and sends his spirit to possess Kakashi. Using Obito's Mangekyo Sharingan, Kakashi aids Naruto and Sasuke to defeat Kaguya.
4 Goku & Piccolo Vs Raditz
During the early days of the Dragon Ball series, Goku and Piccolo were enemies. While they form a strong bond of friendship over the years that followed, in the beginning, Piccolo just wanted to defeat Goku so that he could claim his superiority. However, when Raditz enters the scene, both Piccolo and Goku understand the need to help each other to defeat the third party.
A full-blooded Saiyan and Goku's biological brother, Raditz is a strong opponent to handle. Both Piccolo and Goku use their full strength to defeat him in the battle. While a bit of help is also received from Goku's son, Gohan, it is ultimately Piccolo and Goku's powers that defeat Raditz.
3 Luffy & Crocodile Vs The Marines
Crocodile is introduced as one of the primary antagonists of the Alabasta Arc and Alabasta Saga of the One Piece series. While Crocodile and Luffy are one of the strongest enemies to each other, they have often joined hands to achieve the same goal. Crocodile and Luffy also end up working together to escape from the Impel Down, a maximum-security prison for dangerous criminals.
In the Marinefold Arc, The Battle of Marinefold took place between the Whitebeard Pirates and the Marine Headquarters's Force. During the battle, Crocodile arrives to help Luffy against their common enemy, the Marines. Since Crocodile also wishes for the Marines to lose, he ends up being on Luffy's side.
2 Goku & Frieza Vs Jiren
In the Dragon Ball series, Goku and Frieza are known to be sworn enemies. While Goku is the perfect protagonist of the series, Frieza is a manipulative and ambitious antagonist who will stop at nothing to gain power. Despite having ill-feelings for each other, Goku and Frieza team up to fight against their common enemy, Jiren.
During the Tournament of Power, Goku tries to beat Jiren, however, he is only able to do so with the help of Frieza. The fight is long and dramatic, but it is one of the most interesting fights of the series. While Frieza helps Goku for his own benefit, it still does not take away the fact that Goku needs his help to beat Jiren.
1 InuYasha & Sesshoumaru Vs Naraku
InuYasha and Sesshoumaru are introduced as half-brothers in the InuYasha series. Both of them loathe each other, and Sesshoumaru seems to think little of InuYasha because of his human mother. While InuYasha and Sesshoumaru have often engaged in battles of their own during the series, the brothers come together to beat their common enemy, Naraku.
During the final battle with Naraku, Sesshoumaru and InuYasha use all of their powers to kill Naraku. Since Naraku had ended up threatening the loved ones of both Sesshoumaru and InuYasha, they needed to end Naraku by any possible means.