Supernatural has had its fair share of surprises in plots and side characters. We follow Sam and Dean in their quest, "saving people, hunting things, the family business." To fulfill their quest, they depend on help from other characters. They never hesitate to ask for that help. However, it is not always easy to tell who is on their side.
Characters, such as Cas, have been both the ally and the enemy multiple times. Other characters start out as the big bad enemies, and develop into the reformed bad guys or allies. Meanwhile, some characters start out seemingly on our heroes' side and then switch. It could be said that if the viewer sticks around long enough (such as 14 seasons), the characters they root for will change.
Enemy Turned Ally: Meg
Meg was considered a daughter by old yellow-eyes, the demon who killed Dean and Sam's mom. She supported Lucifer, initially. She tried to kill John Winchester and possessed multiple women. We see the agony experienced by the first real Meg, after Sam exorcises Demon Meg, and the human Meg bleeds out.
Even more, she let out the hell-hounds, leading to both Jo and Ellen's demise. Although she was a playful enemy, she was very solidly an enemy until she wasn't. They unite against Crowley (who surprise, doesn't stay an enemy for long, either), and she develops romantic feelings for Cas. She watches over Cas and appears to really care for him.
Enemy Turned Ally: Death
Tracking down the last horseman is a challenge to do, but Dean has a sit-down meeting with him. Although Dean eventually kills Death in a different season, Death seems to enjoy keeping the two Winchesters around and helps them out. He gives them the opportunity to understand the natural cycle of death when Dean has to act as Death for a short time, reaping lives.
Billie becomes Death, and she also turns into an ally. Initially, irritated about how the Winchesters escape the natural life cycle (they have died and come back many times), she understands their importance once she is Death, rather than just a grim reaper. Based on her dialogue with Jack, it seems that she will be pivotal as an ally in the final season.
Enemy Turned Ally: Ketch
Dr. Ketch, a member of the British Men of Letters, served as a killing machine. When he was ordered to kill, he didn't question. Even Mick, his former leader, wasn't spared when Ketch received an order to get rid of Mick. Dr. Ketch (albeit with help) made Mary a tool to take down the American hunters.
He was not a redeemable character until he tried to prove that he changed. Although both Sam and Dean may not particularly like Ketch, they do now trust him as he is on their side--delivering magical devices, taking down villains, and performing valuable research.
Enemy Turned Ally: FBI Agent Victor Henriksen
The reason that Victor was an enemy was more than he was uninformed rather than a general bad guy. He doesn't know about supernatural beings, and is after Dean and Sam for crimes they didn't commit. He is very driven for justice, but justice means putting the Winchester brothers behind bars, especially Dean.
It isn't until he is in a police station with Sam and Dean, and Lilith is on their trail with her demon minions that he learns of the supernatural world. While out of his element, Victor learns quickly and fights the demons with Sam and Dean. It is too bad that Victor doesn't last long as it would have been nice to have an FBI Agent on the inside, but he proved a good ally for the short term.
Enemy Turned Ally: Amara, the Darkness
Amara is a goddess, eating souls to gain strength and grow. She is God's sister and wants to see the destruction of all that her brother created. He is the light, and she is the darkness. While Dean tries to fight against his connection to Amara, that connection leads to an important resurrection.
After making peace with her brother and going to spend some quality time with him, she resurrects Mary for Dean, sensing that he would be more complete with his mother alive. Because of Amara, we get a few seasons with Mary, and Mary proves to be more than the mother who cuts off the crusts for young Dean's sandwiches. She proves to be a natural hunter and a role model for her sons.
Enemy Turned Ally: Rowena
Rowena has made one of the biggest turns. Not only has she become a better witch, she has become someone who both Dean and Sam frequently call for help. Rowena was introduced to us as an enemy. In search of creating her own coven, she used the women, making them turn rabid, protect her, and then die. She was a character who seemed to be only about her survival, having little love for anyone else, even her own son.
Her horrible death at Lucifer's hand changed her. Although she had spelled herself to return to life, the experience connected her to Sam, who was dealing with his own PTSD from his Lucifer experience. The death of her son also changed her. Rowena, still spunky and fun, became someone that Sam and Dean trusted. She became part of their family.
Enemy Turned Ally: Crowley
Crowley was a fun enemy to begin with and seemed to enjoy Dean and Sam, who he nicknamed Squirrel and Moose. However, he was the King of Hell, and often worked against the Winchesters and Cas.
In the end, he became the only demon in the very inner circle. He died to save the Winchesters. His humor and family drama are definitely missed. This ally needs to return in Season 15.
Ally Turned Enemy: Ruby
Ruby presented herself as an ally; she helped Sam. However, Dean was always suspicious of Ruby. He thought she was a bad influence on Sam, encouraging him to drink demon blood and to practice those powers.
However, she did fix the Colt for them and helped them out of many predicaments. For all purposes, she was an ally. When she revealed her long con, while it wasn't totally unexpected, it was still a shock. The extent of her plan wasn't even known by fellow demons.
Ally Turned Enemy: Anna
It may make sense that Ruby (the demon) was really an enemy in disguise, but Anna (the angel) seemed to be one that they could count on. Not only do Anna and Dean share a very steamy session in the car, but Anna is an angel they can understand. She feels and questions, and she respects what it is to be human.
Choosing to fall, she became a daughter, and she loved her human parents. Sam and Dean protect Anna and help her recover her grace in order to become an angel again. However, Anna doesn't protect them. Instead, she goes back in time to try to kill Mary and John so that Sam and Dean won't be born. She views this as the only way to stop the apocalypse--nothing personal, Winchesters.
Ally Turned Enemy: Ava, One of the Special Children
In the early seasons, Sam discovered that he is one of the special children, fed demon blood as babies. Ava is another one, a seemingly innocent bride, who finds Sam in order to save him since her precognition shows Sam's death. Ava disappears at the end of the episode, and Sam feels guilty. He wants to find her.
When Azazel, the yellow-eyed demon, brings his special children together to Hunger-Games fight to the finish, Ava is there. Sam discovers that Ava has been killing other special children and embraced her powers. She is no longer the ally who would go out of her way to save him; she is the enemy wanting to kill him so that she may survive.
Ally Turned Enemy: Gordon
Gordon is only one of the hunters who turn against Sam and Dean, but he is the most frightening. He views the world in stark black and white, seeing Sam as not just special, but evil. His focus is so unwavering that after he turns into a vampire, he still wants to kill Sam.
Of course, Dean can't allow that to happen, and with Ava's help, Sam is not killed by Gordon. Instead, Sam kills vampire Gordon with satisfaction that frightens Dean. There are other hunters who turn against Sam and Dean (one pair succeeds in killing them), but Gordon is the most frightening in his fanatical way of seeing the world.