In the well-regarded Throne of Bhaal mod Ascension, one way you can prevent Sarevok from betraying you in the final Boss Battle is by using a mod to complete a romance with him. May be downplayed or played straight, depending upon whether you successfully caused him to do a HeelFace Turn.
In Borderlands 2, Krieg the Psycho's motivation for joining the Vault Hunters? The sane side of him is in love with Maya, and thinks that she might be able to, one day, help him act like a normal person again, instead of a rampaging lunatic who constantly screams about riding bicycles made of meat.
Good!Krieg: Now tell her "thank you". Tell her that because of her, we might actually one day be able to act like a normal person again. (Beat) Krieg: (threateningly) I POWDERED MY COCKATIEL FOR THE RIBCAGE SLAUGHTER! (Another beat, then Maya smiles at him.) Good!Krieg: (understandably surprised and delighted at the same time) Heh... Close enough.
Breath of Fire IV: Averted. The Big Bad, Fou-lu, is beginning to nurse a soft spot for the girl that took care of him when he's injured. The girl, Mami, tried holding off soldiers from the Empire that are hunting Fou-lu just so he can escape, and gets captured herself. She's later used as a Sacrificial Lamb for the Nightmare Fueled Carronade, aimed at Fou-lu himself. Even before the cannon was fired at him, Fou-lu feels compelled to Kill 'Em All more than ever...
Dark Souls: Eingyi was a nasty little joker, and is implied to be the reason why the swamp below Blighttown is poisonous. However, after meeting the Fair Lady and her saving his life at the expense of her own health, he happily resigned himself to serving the Ill Girl faithfully and carrying her eggs for her.
Zevran Arainai in Dragon Age: Origins. But subverted for Morrigan, who sees this trope coming and backpedals desperately to stay a bitch... and then double-subverted in Dragon Age: Inquisition: Morrigan offers to surrender without resistance to Flemeth out of love for her son.
Dragon Age IIviciously subverts this with Anders. As his sanity slips away and Vengeance corrupts him further and further, he pleads with Hawke to help him find the ingredients to create a potion that will separate him from his demon side. It turns out that the ingredients are actually components for a bomb, and Hawke's love has made him/her an unwitting accomplice rather than his redeemer. The same thing happens whether or not you romance him, but it hurts a lot more when it telegraphs and then snatches away this trope.
Dragon Quest IV: In the DS remake, beating the boss at the end of the Bonus Dungeon for the first time allows the player to obtain a Yggdrasill flower, which is used to resurrect Rose, the Big Bad's girlfriend. Bringing her to where Psaro was will not only cause him to revert back to his normal self after a nice flashback, but he will join your party.
In The Elder Scrolls series' mythology, this trope falls under the sphere of Mara, the Aedric Divine Goddess of Love. She is a Love Goddess focusing more on the familial, matrimonial, and fertily aspects of love (compared to Dibella, the Goddess of Beauty, who focuses more on the sexual and carnal aspects of it). Her teachings state "those who offer their love to the Divines will never be forsaken." Also covers The Power of Love, as she gave the gift of love to mortals and her teachings indicate that it can change their destiny.
Jaffar in Fire Emblem: The Blazing Blade was a heartless killer until he met Plucky Girl Nino. His newfound ability to feel led him to rescue her from assassins and (if they get enough support conversations) eventually marry her.
A male protagonist can redeem Bastila more easily if they've fallen in love during the game. However, if you fail, she tells you that her feelings for you helped to hasten her fall to the Dark Side. However, if the male protagonist chooses to go Dark Side then him and Dark Side Bastila go on to become a happy little Sith couple, thus subverting the trope again.
Meanwhile, if a female protagonist turns to the Dark Side, Carth attempts to offer her a Last-Second Chance. The official version of the game subverts the trope at this point, as the PC can't be redeemed through Carth's love and the player's only options are to have the PC kill him herself or to allow Bastila to cut him down. A third option cut from the official release, however, would have allowed the PC to be redeemed.
The sequel also contains Visas, who starts as a Sith apprentice and ends up abandoning her Sith ideology if the player is light-side. This is an example of the Power of Friendship if the player is female, though.
Jolee's backstory subverts this trope. All the same, though, he maintains that love will save you, not condemn you; it's unrestrained passion that is dangerous.
In The Legend of Zelda: Breath of the Wild, Impa's bodyguard Dorian was once a member of the evil Yiga Clan before he met the woman who became his wife, starting a family with her afterward. The Yiga Clan retaliate by killing his wife and threatening to kill his daughters unless he spies on Kakariko Village for them.
There's also Thane Krios, although he was aware of this trope before he meets Shepard, with his wife, who dies. It can be presented to him again, if a female Shepard decides to romance him.
Implied to be the reason why Richard Ames allowed Nastasha Romanenko to live through the operation as well as supply evidence against the Patriots in Metal Gear Solid 2: Sons of Liberty's in-game novel: In the Darkness of Shadow Moses: The Unofficial Truth. Suffice to say, he also ended up paying for it dearly in the game when the Patriots decided to off him during the Big Shell Incident.
Neverwinter Nights contains an example near the end with Aribeth. That scene is so similar to the one mentioned in the Knights of the Old Republic section that it seems like they just rewrote some of the dialogue to make it make sense in the Star Wars context, in addition to recycling the character.
Neverwinter Nights 2 has something like this with Neeshka if a male player has enough Influence with her in the ending sequence of the main campaign.
And in Mask of the Betrayer, love is the primary thing that gives people the strength to challenge the monsterous injustice of the Wall of the Faithless.
Radiant Historia: The true ending of the game has the Big Bad Heiss finally accepting his place as the Sacrifice to spare Stocke, whom he truly loved as a nephew, the same fate.
Red Dead Redemption: Played straight and subverted as John Marston's love for his family ultimately leads him to evacuate his wife and son while the army guns him down, knowing that they won't be safe until it's over. Subverted in that Jack was corrupted by the time he spent with John, using the lessons he learned to become a man like his father, exactly what John died trying to prevent.
Ada: Something about the meeting with Leon changed me...
Sacred: One of the Player Characters is a Dark Elf warrior who sets out with a bunch of mates to capture a sacrifice. They end up taking a Wood Elf (also a Player Character) prisoner, but as they're carting her back to her grisly fate, the Dark Elf ends up developing a bit of affection towards his newfound captive. He eventually betrays his comrades and sets her free, before becoming a full-blown Defector from Decadence.
Super Paper Mario: Count Bleck/Blumiere actually ends up saving the world more or less thanks to this. After being reunited with his true love that he previously thought was dead, he pulls a HeelFace Turnthat is powerful enough to recharge the Pure Hearts and break the Final Boss's invincibility.
Super Robot Wars Original Generation give Wodan Ymir Sanger Zonvolt's clone redemption in this manner. He originally was given the mission of protecting "Magus" as a way of tethering his persona and keeping him under control, but when he actually fell hard for her, his creator, Lemon, more or less told him to follow his own will and do what he thought was right. As a result, he does a HeelFace Turn, joins up with Sanger, and together kick an EPIC amount of ass, with Wodan dying, but having the eternal respect of Sanger, the woman he loved protected (and Sanger taking up the mission to do so in Wodan's place per dying request), and he got to die as his own person, not just an enslaved construct.
Virgil from Xenosaga. What makes this interesting is that the person that made him have a case of Love Hurts (Febronia) is also the one that redeems him.