For a video game protagonist to really shine their brightest there needs to be an equally incredible villain against whom they can be compared. Series like Far Cry, Final Fantasy, and the Batman Arkham games are particularly well-stocked in this department, with the likes of Vaas, Sephiroth, and the Joker perfectly contrasting and thus complementing their respective heroes.
Some series, however, like to turn things on their heads, with the bad guys switching sides before the story reaches its conclusion. This is sometimes the result of them seeing the error of their ways or recognizing that there is actually a much bigger bad than them. Whatever the reason, these iconic enemies turned friends have a huge impact on the games and franchises in which they feature.
Shadow the Hedgehog (Sonic)
There have been quite a few villainous versions of Sonic over the years, from Metal Sonic to Silver the Hedgehog, but the most iconic of them all is probably Shadow the Hedgehog. Created by Robotnik using genetic material from the blue blur, Shadow made his debut in 2001's Sonic Adventure 2 and served as one of the primary antagonists for much of the game's main story.
However, toward the end of the game, Shadow comes to realize that both he and Sonic have a mutual foe, so the two team up to take down the Biolizard once and for all. It's one of the coolest boss fights in video game history, with both Sonic and Shadow transforming into their super forms to secure victory. It's not the last time that the two former rivals work together either, with the pair teaming up again in several subsequent games,
Ren (Shenmue)
Players are introduced to plenty of dodgy characters during Yu Suzuki's Shenmue games, but perhaps none more so than Ren of Heavens. The gang leader initially tries to cut Ryo up with a switchblade after extorting money from him, but, after catching a glimpse of the phoenix mirror when it falls from Ryo's pocket, he agrees to help track down Yuanda Zhu in the hopes of finding treasure.
Though this alliance is incredibly uneasy to begin with, the two eventually go on to become good friends. This is interesting to see, as, on paper, the pair couldn't be more different, with Ren being the polar opposite of Ryo when it comes to morals and ethics. It's perhaps these differences that make their interactions so enjoyable to watch though, particularly those toward the end of the second and third Shenmue games.
Metroid (Super Metroid)
The titular creatures in Nintendo's Metroid series are incredibly dangerous and have the power to destroy all life in the universe if they're left unchecked or fall into the wrong hands. It's for this reason that Samus is sent to planet Zebes to recover what is thought to be the last surviving Metroid after it is stolen by the Space Pirate Ridley at the beginning of Super Metroid.
In the first Metroid game and in many of the ones which follow it, Metroids are shown to be enemies of the Galactic Federation and right at the top of Samus' to-kill list. The hatchling she encounters on the Metroid homeworld is a little different, however, as it treats Samus like she's its mother; even sacrificing itself so that Samus may live during her second fight against Mother Brain.
General Leo (Final Fantasy 6)
Though its ridiculously large cast of playable characters and the fact that so many of them are optional arguably limits opportunities for genuine character development, the overarching narrative of Final Fantasy 6 is one of the best that the long-running JRPG franchise has to offer. It has a little bit of everything, including a healthy dose of side-switching and betrayal.
After realizing that he's actually one of the bad guys General Leo decides to switch sides and team up with the Returners in a bid to bring an end to the Gestahlian Empire's plans to conquer the world. Sadly, Kefka ends up killing Leo almost immediately after his heel-face turn, but Leo's change of allegiance is an important one nonetheless. It helps players to come to realize who is really pulling the strings and kickstarts a catastrophic chain of events that tears the planet into pieces.
Pokemon (Pokemon)
With the exception of starter Pokemon and those which are traded with friends or NPCs, pretty much every Pokemon that players add to their teams starts out as an enemy. Of course, some Pokemon are more aggressive than others, with Pokemon Legends: Arceus showing that Pokemon are more than happy to attack a trainer if they don't have a Pokemon protecting them.
Over the many generations of Pokemon games, players have seen countless hostile Pokemon become their friends, ranging from low-level Caterpie and Weedle to powerful Legendary Pokemon like Mewtwo and Ho-Oh. It could perhaps be argued that wild Pokemon are not enemies in the traditional sense, but, for all intents and purposes, the fact that they're more than happy to attack a trainer and their Pokemon until they lose consciousness must surely put them in the same bracket.