Marjory Delaqua and Kasmeer Meade in the living story for Guild Wars 2. In the living story season 2, your character is joined by a small party of adventurers which includes this duo, and a variety of battles and adventures are had.
Gordon Freeman and Alyx Vance from the video game Half-Life 2. While the exact nature of their relationship isn't clear (Gordon never speaking makes things difficult), whatever it was, it was forged in the fires of battle.
Raider and Raven. It doesn't make for any huge change as gameplay is concerned (they merely share moral gains.), but they do have their own Informal Eulogies for each other if one should get killed in battle.
There's also Wolf and Fox — she's the reason he lost all that weight.
In Metal Gear Solid 4: Guns of the Patriots, Johnny eventually wins Meryl's heart by saving her life on not one but three occasions. In this scene, they hold a door against endless waves of attackers while slipping new magazines into each others guns, and proposing each other for marriage.
In Metal Gear Solid 3: Snake Eater, the Boss and the Sorrow had such a relationship before suffering a fate much worse than Star-Crossed Lovers - being forced to fight each other to the death in order to uphold loyalty to their respective nations, namely the US and the USSR. Said fight ended in the Sorrow's death. However, before being forced to split, the Boss gave birth to The Sorrow's son (Revolver Ocelot) while participating in the D-Day invasion. Later, it turns out that the Sorrow encouraged the Boss to kill him to complete her mission. This strongly implies that the Sorrow wasn't willing to fight to defend himself against her, showing that his loyalty was first and foremost to her.
Pick a Fire Emblem couple. For characters to develop a relationship, the gameplay requires them to stand close on the battlefield a certain amount of time. You get bonuses added to their stats when they do this. With Min-maxed supports (for example 2 earth alignments for hax evade) the units in question are able to take on half the battle by themselves. The Power of Love (or friendship), is indeed a mighty force.
The most obvious examples, both story- and gameplay-wise, might be Louise and Pent from The Blazing Blade (a Happily Married couple who start out with an automatic, unbreakable A-rank support) and Micaiah and Sothe from Radiant Dawn (who, once again, start out with an automatic A-rank support and will marry at the end if the player doesn't break up their support).
Brought to new highs in Fire Emblem Awakening. Now two characters can go together (as in, sharing the same tile), which gives the active partner a boost in stats and allows their Relationship Values to develop faster. All male/female team-ups (except brother-sister or parent-child relationships, pairings involving a generation gap excluding those with the Avatar, and certain pairings involving Chrom or Sumia—who can only marry a handful of characters—or Flavia, Basilio, Anna, Say'ri, Tiki, or the SpotPass characters—who can only marry the Avatar) can potentially end up in marriage.
This is half the point of the Ar tonelico games. Inevitably the main character (who is a melee fighter) is paired with a backline magic-using woman. When blocking attacks against her, he stands very, very close. When she is casting spells, their power is increased by her love for him. In the second game, the combat dialog is full of innuendo.
Sonic and Amy zigzag this trope depending on the game.
Sonic Advance 3 is a parody because they are referred to as a "Lovely Couple" even though Sonic doesn't look too happy about it, while Amy wishes they were.
Downplayed in Sonic Rush where Sonic and Blaze fight against Eggman and Eggman Nega. Despite the occasional Ship Tease, it's up for interpretation whether there is anything between the two.
Depending on how you look at it, Rouge could very well be this with Knuckles in the Sonic X adaption, or sometimes in the games themselves. Or she could be this with Shadow, due to them working together so well.
Dante and Lady in Devil May Cry 3, true Dante is flirtation is mostly unrequited with Lady spurning his advances saying she's not desperate enough to date a demon... but just look at em. There's some Belligerent Sexual Tension at the very least and they do work well together, even giving Vergil a hard time.
The Cool Couple double battle trainers in the Pokémon Ruby and Sapphire games, and the Double Team trainers, the Generation IV version of the Cool Couple (a pair of Cool Trainers or Ace Trainers, respectively). They make it in the anime as Oscar and Andi. See Anime.
Halo: Hunters. You know those giant, pulsar-cannon dual-wielders that always come in pairs? The ones that basically have the same combat rating as a Spartan by sheer badass alone? They're always brought into combat in multiples because they're divided by mating pairs formed by their Hive Mind worm colony bodies, so if you kill one then you've actually killed off the entire roster of girlfriends for the hundreds of worms in the surviving unit, and they're collectively pissed off at whichever unfortunate Spartan did the deed.
South Park: The Fractured but Whole has Super Craig and Wonder Tweek who had intended to be a battle duo. Due to their factional split, they are described as "dating but fighting". If the player helps them mend their relationship, they gain the "Eros Eruption" Limit Break that lets them invoke The Power of Love to deal damage. Their abilities in-game also compliment each other, with Craig's being focused on drawing aggro and dealing heavy close-range damage, and Tweek's being focused on inflicting status effects and healing.
Baldur's Gate series has a few. There's Khalid and Jaheira, and to a lesser extent, Eldoth and Skie, in the first game. In the second game, the player him-/herself could have one, with one of the several options, while Haer'Dalis and Aerie, and maybe Korgan and Mazzy, have potential for this as well (but in an unmodded game go nowhere).
The possibilities for this in the Knights of the Old Republic games are endless... In the first game, the Player Character and Bastila (especially awesome if you decide to join Bastila on the Dark Side, or alternatively heartwarming if you can use The Power of Love to return her to the light side) if you play male. Carth or even Juhani if you play female. You get a choice in the second- The Exile and Atton or the Disciple if the Exile's female, and Visas or the Handmaiden if he's Male.
Star Wars: The Old Republic has these all over the place. In the Backstory, there's Eleena Daru and her "husband" Darth Malgus. There was also Satele Shan's fling with Jace Malcolm (that produced Theron Shan). Each player class has a companion with a romance arc that can and will be brought into a fight with them. Out in the expansions, there are even more options, like Theron, Lana Beniko, and Koth Vortana. In BioWare tradition, all of their romance arcs can be accessed by a male or female player character.
As above in Neverwinter Nights: Hordes of the Underdark, with the ghost of Aribeth or Nathyrra and your PC, who end up beating up Mephistopheles. Or Valen and the PC if the PC is female.
Neverwinter Nights 2 (Though it was made by Obsidian) kind of precludes this with three of the four love interests over the course of the official campaign/Mask of the Betrayer arc being spellcasters (and Safiya being a literal Squishy Wizard), but a female PC and Casavir qualify. A male PC and Elanee, if she is optimized for combat; or a female PC and Gann if he is optimized for combat both fit.
If the player romanced neither, Garrus and Tali get together in 3.
There are a couple of aversions on Shepard's list of potential hookups, however. Kelly Chambers and Samantha Traynor are never seen going into combat, with Traynor in particular having a long list of reasons why she should stick to her day job rather than heading for the battlefield. Steve Cortez is an interesting case, in that while he never fights with Shepard, he's his dropship pilot, and has dialog with him on missions (especially the Leviathan DLC and the The Very Definitely Final Dungeon.)
Any romance option in Dragon Age: Origins. Becoming a couple will help the characters level up.
Especially long-lasting example is Leliana: in the Awakening epilogue (only if you romanced her and survived until then, of course), it is mentioned that the Warden is still out there adventuring together with her, years after the events of the game.
In the third game a Romanced Leliana is still together with the Warden, just temporarily split by the current crisis. Similarly if a romanced Morrigan and the Warden stayed together in Witch Hunt, they've been raising a son together in the interim and she leaves to rejoin him at the end of the game. A romanced Zevran is mentioned to still be with the Warden on their current adventure.
If romanced, Alistair and the Female Human Noble can eventually become the King and Queen of Ferelden. Their bond is so strong that Alistair will refuse to let the player do an Ultimate Sacrifice, and kill himself in their place. In a game where every choice matters, this is the one choice Alistair will not let the player choose.
In Inquisition, it's noted that they're still a couple, but she's out searching for a cure for the Calling, since it's been ten years since her Joining and Grey Wardens are all Living on Borrowed Time. He misses her, but understands that she's doing it for both their sakes, and supposes that someone has to rule the kingdom; can't have all your royalty out on a dangerous adventure, now, can you?
Likewise, any of the romance options in Dragon Age II, plus Aveline and Guardsman Donnic if Hawke takes the time to help them get together. Aveline is also this with her first husband Wesley when you meet them.
The tactics options allow you to turn any two of your companions into one of these as well. If you ship a particular pairing, you can go into the tactics settings and arrange it so that they will defend each other on the battlefield; if one is a rogue, you can add the Back to Back ability to make it even more visually obvious.
Iron Bull and Dorian can become a couple if neither is romanced by the Inquisitor and are placed in your party enough during the game, providing one of the few non-PC/squadmate examples of this trope in a BioWare game.
Seeing as in the game's world any participant in the Reapers' Game needs a partner or they're powerless, it can be assumed that there have been quite a few Battle Couples out there.
One other case stands out in particular: Uzuki Yashiro and Koki Kariya. This Reaper duo fights you twice in the game and even have their own version of a light puck as well as a team attack that can render players that are under-leveled at those points in the game, screwed. Their relationship can also be considered Vitriolic if not an outright Battle Couple.
There's Neku and Shiki, who are treated as the star couple in all but name. Sure, they had a rocky start, but by the end of their chapter they've become an unofficial Battle Couple, such that their separation is a major tearjerker for either of them, while their reunion at the end of the game is all the more heartwarming.
There's also the two-person Espada Squadron in Ace Combat Zero, whose Number Two heavily implies that she was in a long-time relationship with Espada 1 at the time of their fateful encounter with the Galm Team.
Even earlier, Ace Combat 04 gave us Yellow 13 and Yellow 4, the only permanent members of their squadron. However, in their case, the romantic feelings were probably one-sided, since 13 seemed to have only seen a valued student and a trusted wingwoman in 4.
Just about every couple in the Super Robot Wars Series is this. But especially the ones that have a combo attack.
City of Heroes: Fusionette and Faultline. She's a mutant with a nuclear reactor for a heart, and he's the son of the guy who wrecked Overbrook. It turns out the guy was Brainwashed and Crazy. You run into them repeatedly in the Rikti War Zone with them kicking butt all the way.
Manticore and Sister Psyche, teammates in the Freedom Phalanx who eventually got married... and had to fight off a horde of PC villains at their wedding.
The various installments of Dynasty Warriors gives us Lu Bu and Diao Chan, Cao Pi and Zhen Ji, Meng Huo and Zhu Rong, Sun Ce and Da Qiao, Zhou Yu and Xiao Qiao, Liu Bei and Sun Shang Xiang, and Zhuge Liang and Yue Ying.
Dynasty Warriors Strikeforce has several missions based entirely around fighting said Battle Couples.
If you get the best ending in Cave Story, it becomes clear that there's a bond between Quote and Curly Brace. It's not clear if it's platonic or romantic, but either way, it's strong as iron.
This trope is the concept behind the Sakura Wars series. The games are strategy-RPGs where the stronger the bond between you and your teammates is, the better you all perform in battle.
Atelier Iris: Eternal Mana has Klein Kiesling and Lita Blanchimont as the Galgazit partners with an inversion of the usual hero and heroine state as Lita is a tank while Klein is a magician/healer.
Lololo and Lalala from the Kirby series. Justified in the anime adaptation, as they're two halves of the same being.
Isaac and Jenna from Golden Sun married after the period they fought together. Considering how long it took for their kid to pop out, they probably did not spit it out until some time after the first series ended.
The implication is that the same applied for Felix and Sheba, made more poignant by the fact the two get each other out of tight spots - Felix saves Sheba's life, and Sheba is the only reason the party could even contend with Jupiter lighthouse, since she needed to become the hero of Shaman Village to gain the hover jade necessary to traverse the place.
A strange Foe Romance Subtext example: Alvin and Presa are hinted at to have been this, or were on the verge of becoming so, in Tales of Xillia, but their relationship descended into a love/hate one by the time of the game. Jude and Milla, however, are confirmed to have developed into one by the game's ending although, sadly, they can't stay together and she has to go for an Always with You option. Milla and Jude also form one by the end.
Final Fantasy XIII-2 is an aversion. While Serah and Noel fight together and grow close over their adventure they do not become a couple. They have other love interests in Snow and Yeul so one can say they are a Battle Platonic Partnership.
Final Fantasy XV: Noctisand Luna counts, even though their only time to shine is when they fight against Leviathan. They are not even in the same party, as Luna empowers Noctis from afar and got killed from her knife wounds caused by Ardyn afterwards. The cancelled expansion-turned-novel Dawn of the New Future expands this further as they fight alongside basically everyone against Bahamut.
Fox McCloud and Krystal from Star Fox. Shortly after Fox freed her and defeated General Scales, Krystal joined the Starfox team as one of Fox's wingmates, and the two began a relationship. In Star Fox Command, said relationship hit a rough patch when Fox decided he didn't want Krystal in harm's way, though depending the ending they patch things up. Or not.
Freed and Yoshino Yamamoto (Suikoden II) also get a Unite Attack, the "Husband and Wife Attack", if they're put in the party together. Though Yoshino proves to be a much better fighter than her husband, who has mediocre stats all around and is more of an administrator than a fighter.
Similarly in this game, the Prince and his bodyguard Lyon. Save for an event or two forced by the story itself, you can't remove Lyon from the party for most of the game. There's an element of Like Brother and Sister given that Lyon was Ferid's ward, but given that the Prince also has a full blood sister, and Lyon's not nearly as close to her, it's pretty obvious, particularly toward the end of the game.
Gladius has a cup where you deploy 1 male (the only time a male unit must be deployed for their maleness) and 1 female unit and claims to be based on this. Nothing stops you from deploying the twins Ursula and Urlan beyond not complimenting each other (both being medium class units), presumably because its made by LucasArts.
The Legend of Zelda: While Link and Zelda have supported each other in battle several times throughout the series, it has almost always been as Lady and Knight. The closest they've ever come to being in a confirmed relationship is in Spirit Tracks, in which they're pre-teens, making their relationship closer to Puppy Love. In Breath of the Wild, they're both 17 (though chronologically 117). The travelling minstrel Kass explains that his teacher was envious of Link because Princess Zelda "only had eyes for her appointed knight". But in the western version, Link's own feelings toward Zelda are deliberately left ambiguous.
Sasha Nein and Milla Vodello of Psychonauts, a pair of psychic secret agents who work on the field as partners. Their personalities are as opposite as it gets, but they clearly think highly of each other and have a long, fond history together - that said whether or not they're an Official Couple is no more than implied (Milla has a Memory Vault showing pretty obviously that she's in love with Sasha, and he seems to reciprocate in it, at least to some extent, though other vaults in the game make it clear that such vaults' contents aren't necessarily objective).
Rune Factory 3 allows you to pick up a partner and go into battles. If your partner is your love interest or wife, it's this trope. Sonja and Aden from Rune Factory Oceans may or may not count, since they share the same body for a large amount of gameplay.
In The Elder Scrolls V: Skyrim you can be part of one yourself. This is because you are able to recruit an NPC to fight with you and ask them if they want to marry you. Do both and you play this trope straight.
There are a few examples within the game itself as well, such as Bujold and Kuvar in the Dragonborn DLC. Aela and Skjor of the Companions are rumored to be this, but Aela insists they're Just Friends.
Like the above Skyrim example, in Fallout 4, you can romance your companions and have them fight alongside you.
Dart Feld and Shanna of The Legend of Dragoon are adventurers that can transform into Dragon knights. They become this trope after Shanna admits her feelings at the end of disk two.
Persona: In Persona 3, Persona 4 and Persona 5, you can get a girl or boyfriend of the opposite sex, including your party members, resulting in you fighting enemies alongside your lover. The only exception is Junpei in Persona 3 Portable, who can't be romanced due to Chidori stealing his affections. Persona 2: Innocent Sin also allows for this, with the notable addition of being able to form a same-gender battle couple.
Nate and Elena, the Official Couple of the Uncharted series, get together while fighting off an army of Mooks and hunting for El Dorato on an uncharted island, which set the tone for their whole relationship. Uncharted 2: Among Thieves briefly has Nate and Chloe, during one of his frequent break-ups with Elena.
StarCraft II continues the story. After despairing for years on his guilt about what happened to Kerrigan, Raynor finally succeeds to de-infest her and bring her back to her original personality... only for her to turn herself back into a Zerg willingly so she can go on a Roaring Rampage of Revenge against Mensgk. While she retains control this time, and even saves him, he is understandably horrified by what she has done and put an end to their relationship. Eventually he realizes Kerrigan is no longer the blood-thirsty monster she was as the original Queen of Blades, and comes to save her in the final confrontation with Mensgk. By the end of the game, Raynor and Kerrigan are on good terms again, and while they cannot stay together due to consequence, they clearly still love each other.
StarCraft II also introduces Mira Han, an eccentric mercenary leader and pirate, and Matt Horner, a Boy Scout war buddy of Jim Raynor who unwittingly won Mira's hand in marriage in a poker game. She's much more in to it than he is, and while he'd rather have nothing to do with her, he's not above taking her help when he needs some extra firepower. The two even work together as one of the game's Co-op commanders; Mira provides cheap, easily replaced mercenary units, while Matt supplements them with powerful but expensive Dominion aircraft.
The flying wyverns Rathian and Rathalos are implied to operate like this. They're actually the female and male of the same species, and their descriptions often refer to them hunting together as a couple. There are even a few missions that pit players against a Rathian and Rathalos together. In World, if both are present on the same map, they will even come to each other's aid and protect their wounded counterpart.
As of Monster Hunter: World, Teostra and his own female counterpart Lunastra can now get in on the action. Both are Elder Dragons, which makes fighting both at the same time even more epic.
Orcs in World of Warcraft hold this as an ideal. In particular, in Warlords of Draenor, we are introduced to Durotan and Draka, the orcs who would become Thrall's parents. If the two aren't side by side in battle it's only because one is defending the home front while the other goes out on a mission. In an in-game conversation during the Frostfire Ridge quests, Draka is shocked that Thrall insisted his own mate Aggra Stay in the Kitchen rather than be at his side in war, where she insists it's most important to have your family by you. Aggra later appears in Nagrand, having set Thrall straight on that point herself.
And let's not forget the Night Elf leaders, Malfurion Stormrage and Tyrande Whisperwind.
Actually there are a lot of these in Warcraft, because the world is kind of really dangerous so most people have to fight sometimes. Warcraft has Arthas Menethil and Jaina Proudmoore, before the former does a Face-Heel Turn. Although they fall into the fighter and mage archetypes, it's a slight subversion of expectations because Arthas has the healing abilities (in RPGs, the girl always heals). Jaina's abilities mesh particularly well with Arthas' because they revolve around slowing down her opponents (which makes his job, of smashing them into bone dust easier). In Wrath of the Lich King Jaina tries a last-ditch attempt at persuading Arthas to resist the influence of the Lich King nearly getting herself and the player killed. You can also loot a locket that used to belong to Jaina from the Lich King fight, proof that Arthas kind of still loved her (maybe). The expansion Legion also formally introduced us to Turalyon and Alleria, who are the Mamma Bear and Papa Wolf to basically the whole world (but most importantly, their son Arator who has become quite the paladin himself). Sylvanas and Nathanos Blightcaller were, and still kind of are, this. They were definitely lovers when they were alive, and they still act like it now even though they both don't like to admit it. Rhonin and Vereesa were this, kind of mixed with Ruling Couple as well since they were the de facto leaders of Dalaran's military. Oh yeah: Sylvanas, Alleria and Vereesa are sisters. Their parents were probably also this although their mother is never explicitly mentioned. The titan keeper Thorim and his Vrykul girlfriend Sif. The Yogg-Saron later uses Sif as a disguise to manipulate Thorim.
Sands of Destruction: Kyrie and Morte become one about halfway through the game. He dislikes fighting and fell in Love at First Sight, while she's a Blood Knight and takes a while to realize that, yes, he's more important to her than just another tool in the World Annihilation Front box.
There's more than one in Mortal Kombat, with the most glaring examples being Liu Kang and Princess Kitana (who mix it with Star-Crossed Lovers until X, when they become the Netherrealm's Ruling Couple) and later Kenshi's son Takeda and Jax's daughter Jacqueline "Jacqi" Briggs in Mortal Kombat X (who rescue each other mutually several times in the story).
Ferra: Pretty boy. Takeda: Sorry, I have a girlfriend.
The second boss of Ninja Baseball Batman, Mad Lax, could count as this, as driving the jeep are two anthropomorphic baseballs, one with a bow on her head, implying the two are a couple.
Two of the Royal Guard members you have to fend off are a married couple, Dogamy and Dogaressa. They have matching his-and-hers sentry stations — labeled with actual signs saying "His" and "Hers" — and they keep touching noses and "saying sickly sweet things to each other", as the flavor text states, while they fight you.
Then there's the team of RG 01 and RG 02, who have Unresolved Sexual Tension to spare and can be persuaded to admit their feelings for each other.
Rakan: I love watching you kill. Xayah: I love killing for you. Rakan: Let's make out.
Not uncommon in Xenoblade Chronicles 2, thanks to the need for trust and cooperation between Driver and Blade. One NPC mentions that in older times it wasn't unheard-of for Drivers and Blades to marry. She brings this up about Rex and Pyra, who are a shining example of this trope to everyone but themselves.
This gets deconstructed hard in the Torna prequel DLC. Jin, already under a lot of stress from learning about his past (where he was part of a battle couple with a woman implied to have actually married him) and seeing the full might of the Aegis, is made to consider the full implications of being a battle couple with Lora once it's clear to both of them there's some degree of love between them both. He decides he cannot live without her (which Blades literally cannot do) and takes drastic measures when Lora starts to succumb from terrible wounds that end up putting Lora into a terrible coma.
The best example would be Bai Jinjin and her boyfriend Zhong Lizi, with Jinjin as the model/frontline fighter and Lizi as the stylist/strategist.
Royce and his Beleaguered Assistant Neva would fit in nicely... if he wasn't constantly complaining about her presence scaring away potential models, and if she wasn't a Reluctant Fanservice Girl who thinks that modeling his designs isn't a part of her duty.
Vlad and Isabella von Carstein in Total War: Warhammer have this as a gameplay mechanic: already quite strong alone, the two Legendary Lords gain massive bonuses to their stats when fighting together in the same battle. In addition, by starting a campaign with either vampire, you can recruit the other right off the bat, and at a discount too (in other campaigns you need to fulfil a certain criteria to unlock the Lord you didn't start with).
Haven (2020): Kay and Yu fight together, and their best attacks are combos where they work together to blast the enemy.
Yes, Your Grace: Asalia's main Tomboy Princess quality is that she gets really into weapon-wielding when she gives it a try, and her true love is already experienced in that domain. If the two of them end up together, they will go on to form a combat-ready pair.
In Dark Devotion, the Dual Boss Elinor and Aureus have fought alongside one another in service to their king for decades and are implied to be romantically involved. When you defeat them, Elinor will limp over to Aureus's side so they can be Together in Death.