It's a classic trope, isn't it, when a baddie decides that a life of crime isn't for them and they join up with the heroes? It's a great way to introduce some conflict into the party, redeem a dark horse, or simply throw a wrench into the works.
In other cases, it can provide you with a brand new party member or some other cool perks, or maybe a simple satisfying payoff for taking the steps to turn somebody onto a lighter path. No matter how it's done, it's usually the highlight of a character's narrative arc.
10 Edea Lee
Edea is a very quick conversion in Bravely Default. She begins the game on a mission to capture Agnes, the vestal of wind who is naturally also one of your party members. It only takes a few moments of exposure to the horrific methods of war that her comrades employ for her to flip over to the good guys.
Edea's storyline is quite a compelling one as your journey will pit you against her ex-allies and even her old fencing teacher. Her internal struggle is one of the highlights of the game.
9 Magus
Chrono Trigger is one of the most well-regarded classics of the RPG genre and it's easy to see why. As a result, the fact that you can redeem Magus and have him join your party is something of an open secret.
While it may be more satisfying for some to see Frog fell his archenemy, Magus makes for a useful party member with lots of elemental spells and a really snazzy scythe.
8 Aria T'Loak
Omega is a gigantic mining station that serves as a hub for all sorts of villainy, black market acquisitions, and general criminality - and Aria is the head honcho. She's a ruthless Asari with incredibly strong biotic powers and she isn't afraid to use them, it seems.
In Mass Effect 3, Omega is taken from Aria by Cerberus and she plans to take it back by any means necessary. A Shepard with a high enough Paragon score can convince Aria to form a less dangerous plan in the spirit of reducing the loss of innocent lives, to the relief and joy of said innocents. While she may end up the most angelic character, she does solidly support Shepard and their cause in return.
7 Milich Oppenheimer
Milich Oppenheimer is a narcissistic, flamboyant, and dangerous general of the Scarlet Moon Empire in Suikoden. He is about as villainous as villains get and is responsible for the death of Gremio, a surrogate father for Tir, the main character.
Later on, it is revealed that his villainy was due to the influence of a Black Rune given to him by the game's main antagonist. When this is learned it is possible to forgive Milich for his crimes and recruit him into the party where he serves as a very capable magic swordsman. Alternatively, you could just avenge Gremio and execute the disgraced general.
6 Gabranth
Final Fantasy 12 has a plot absolutely dripping in political intrigue and backstabbery, and Judge Magister Gabranth is quite the key player. His crimes are many, most principally assassinating King Raminas while disguised as his brother Basch, framing him.
It takes until the very end of the game for Gabranth to get his first steps on the path to redemption. He is ordered to kill Larsa, a boy he has spent many years protecting and is obviously very fond of - this order turns him against his ruler and he joins the party in a boss fight against Vayne. This, unfortunately, results in his death. At least he made the right decision eventually, right?
5 Axel
Kingdom Hearts veterans will know that villains becoming heroes is an absolute staple of the series, though Axel is one that sticks out in many players' minds. His story arc in Kingdom Hearts 2 is a very emotional one, with many of his actions being driven out of a sad desire to see his friend Roxas once more.
After being defeated by Sora, Axel spends his last few breaths to apologize to the keyblade-wielder and opens the path to the World That Never Was. Luckily for him, this isn't a final death and he appears in pretty much every other game in the series. Because of course he does.
4 Staunton Vhane
As a CRPG, Pathfinder: Wrath of the Righteous obviously has a lot of branching points in its story where the player's decisions can make huge differences. In this game, one of these decisions is which Mythic Path you should take, with each one having different quests and perks associated with them.
Becoming an Aeon is one such choice and allows you to take the drastic action of going back in time to redeem the disgraced Staunton Vhane. While usually Staunton is a tragic villain with few reasons to empathize with him, this path turns him into a decently useful party member (without having to raise him from the dead).
3 Zevran
Early on in Dragon Age: Origins, you will get ambushed by a group of assassins led by a roguish charmed named Zevran. It's quite a quick and easy encounter that makes you wonder whether these "Antivan Crows" can really be as efficient as they claim to be.
Nevertheless, this encounter ends with your character confronting a tied-up Zevran and it's possible (and recommended) to get him to join your party. He makes for a delightful addition to the crew and always has something witty to say, in case you want to lighten the mood.
2 Silver
Silver is a standout example of a compelling character in a Pokemon game from a time when the games didn't focus on any sort of narrative. He begins the game as a petty thief and becomes your rival, constantly insulting you and your Pokemon regardless of whether he wins or loses.
It is only after you have beaten the Elite 4 that Silver's deeper goodness comes out. He admits that his past doings are wrong and he obviously starts to really care for his Pokemon, which can be evidenced by the fact that his Golbat becomes a Crobat - something that can only happen when the Pokemon is very happy and attached to its owner!
1 Beatrix
If you want a satisfying redemption arc, look no further than Beatrix from Final Fantasy 9. When first met by the party, Beatrix is shown to be a confident, ruthless woman who simply cannot be beaten in a fight. She will take the party down three times before she sees the error of her ways.
It is when she realizes that the queen that she has been loyal to turns out to have no love nor care for her daughter, Garnet, that she relents. It doesn't take long for her to switch allegiances and turn against Queen Brahne and her jesters, and she remains a reliable and powerful ally until the end of the game.