There are countless partnerships in comic books, anime and films of characters who are on again/off again friends and enemies. They start of as hero & villain and become comrades or even start off as comrades and slowly drift apart and it becomes an antagonistic relationship. Whatever the situation may be, it's these relationships that draw us into their story.
We probably even have those people in our own lives that started out as a friendship but then something happened and eventually our common ground faded and that friendship may have even turned sour. Let's take a look at some of the more recognizable frenemies.
Naruto / Sasuke
Any fan of this anime knows the back and forth that these two have gone through throughout the series. As complicated as a relationship can get due to the ancestral feuding dating back to the beginning of the Shinobi. At a young age when they were both just young Academy students, Naruto wanted to be friends but slowly his attitude turned to jealousy as Sasuke proved to be better at everything. A rivalry was then formed as Naruto vowed to equal Sasuke. They were then put onto a ninja-squad team and trained together. Their rivalry became more friendly as they acted like brothers off and on through the following confrontations, eventually growing to be best friends and counterparts to each other.
Captain America / Iron Man
We've all seen it coming, the Civil War, the battle to end all alliances. A wedge will be driven between these two heroes as we watch Iron Man struggle to find structure and order and attempts to manage all super powered individuals through the Government; and while Captain America struggles to fight for the freedom of all to live outside of the thumb of a over-arching government power. The two fight and bicker, but they respect each other at the same time. Their rivalry is on a more grandiose scale as a civil war between their teams breaks out because of their diverging ideologies.
Goku / Vegeta
This relationship is the epitome of frenemies in my mind. Goku and Vegeta's relationship started off as the starkest of adversaries. Vegeta being the prince of the Saiyan race and Goku (whom he knew as Kakarot) was merely a third class warrior unworthy of his time or attention. Vegeta's indifference turned to hostility after being bested by Goku and his son Gohan. His pride destroyed, he vowed vengeance against Goku. They fought continuously as Goku kept surpassing Vegeta adding to his hatred until he (Vegeta) near death was called 'friend' by Goku, and his hatred vanished.
Xavier / Magneto
Two incredibly powerful mutants who over time grew apart, but still ultimately hold a great deal of respect for each other. Their friendship started off on the right foot as the two men enjoyed debating each other and sought to bring legitimacy to mutant-kind across the world. This seemingly shared interest in supporting other mutants is where the line was drawn between the two: one sought to meld mutants into society peacefully and the other to dictate the future of the known world on behalf of mutants.
Thor / Loki
Brotherly rivalry is normal; fighting is just as normal between brothers. Thor and Loki came of age together, they fought many battles together. Eventually Loki's true self was revealed and he discovered his real gift as the God of Mischief, and their once brotherly relationship became increasingly strained.
Spider-Man / Green Goblin
Two young students who started off as a casual friendship. Harry would help keep Peter's tormentors off his back, and Peter would in turn help Harry with his homework. Eventually the pair would become best friends. ...At least until Harry's dad entered the picture as Spider-Man's archenemy The Green Goblin. When Norman (Harry's father) died, Harry's mind was broken and he blamed Spider-Man ultimately becoming The Green Goblin himself.
Gray / Natsu
Partners in combat even though both consider the other to be their main rival. Natsu and Gray are guildmates in the Fairy Tail guild of magicians. They have fought with and bickered with each other since a young age. Despite the constant battle between each other, the two share a strong bond, nearly brotherly. They have always had each others back, saving the other from either being defeated or sacrificing themselves to defeat whatever challenge they were facing.
Ichigo Kurosaki / Uryu Ishida
Initially introduced as mortal enemies. Ichigo being a soul reaper and Uryu being a quincy. There is history between Reapers and Quincy that predates the two characters. They are fighting an eternal battle between factions as both believe the other to be their mortal enemies. Uryu specifically blames the Soul Reapers for not coming to the rescue of his grandfather and mentor when he was attacked by Hollows. Ichigo and Uryu have teamed up on several occasions even though their relationship has been strained at times, they have a mutual respect for each other.
Wolverine / Cyclops
These two have been at each others throats more times than we can probably remember, but they have also partnered up when it truly all comes down to it. They have an unhealthy competition over the affections of Jean Grey. Cyclops is the pronounced leader of the X-Men and although Wolverine follows his lead, he doesn't always agree with Cyclops adding to the tension between the two. Ultimately the two respect each other, but at the same time would gladly choose to never work together again.