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An Ode to AI Sidekicks

‘The Last of Us Part II’ reminds us that sometimes there’s nothing better than a competent, personable, computer-controlled sidekick

Sony Computer Entertainment/Sony Interactive Entertainment/Valve Corporation/Irrational Games/Ringer illustration

Ellie, a playable protagonist of The Last of Us Part II, is a tough teen to love. She’s got a great sense of humor, strong survival skills, and a unique immunity to the Cordyceps fungal infection—all pluses in a postapocalyptic America populated by cultists, mercenaries, and monsters. There’s just one slight personality … well, let’s call it a quirk: She’s hell-bent on violent revenge, no matter how many hundreds of humans and dogs she has to slaughter or torture along the way. Your mileage may vary, but for me, murderous rampages are a real relationship red flag.

That minor murder hang-up makes it hard to see eye to eye with Ellie; sometimes it’s just like, “We snuck around swamps and slit soldiers’ throats yesterday, can’t we do something different today?” But although we didn’t always agree on whether to, say, head back to town and spend our days in peace and safety or venture into Infected-filled buildings and beat people with pipes, there was one moment when we vibed. Ellie keeps a journal in which she scrawls some of her thoughts as the game goes on. In one entry, tucked amid the doodles, dark poetry, and death threats—your usual adolescent angst—she writes a brief tribute to Jesse, a friend who fights alongside her during parts of her quest. If I had been journaling, I would have written something similar.

Screenshot via Sony Interactive Entertainment

“Jesse’s here,” she wrote. “This is good. He can help protect Dina. It felt good having him around to deal with the WLF.”

Ellie was right: It really did feel good having him around. Jesse is generally a cool customer, an easygoing guy who helps his friends and, unlike almost everyone else in the game, doesn’t have a death wish or a lethal score to settle. The Last of Us Part II is a trying, traumatizing experience. For extended stretches of the campaign, Ellie and opposing playable protagonist Abby travel alone, which seems scary and reckless. After each of these solo stretches, I was desperate for the company of a character who wasn’t trying to kill me—ironically, someone like the younger Ellie, who endeared herself to players of the original The Last of Us as a mostly computer-controlled character. When Jesse showed up at the end of an especially harrowing sequence, I could have kissed him, and not just because he’s a postapocalypse 10.

Jesse and a suite of several other nonplayer characters (NPCs) who accompany the protagonists of TLOU2 (including the equally capable and likeable Lev) are prime examples of one of my favorite figures in video games: the competent, personable, computer-controlled sidekick. AI companions provide friendly faces in a hostile environment. They assist in navigation, puzzle-solving, and combat. They alleviate loneliness, enable banter and teasing, and facilitate character development. And because they have so many tasks to perform, they’re almost impossible to implement perfectly. When they work, though, they elevate interacting narratives and deepen the player’s emotional ties to the game.

“One of the most interesting and enduring innovations of games is that feeling of ongoing copresence with an NPC, doing and being together in the gameworld,” says author and researcher Katherine Isbister, a professor in the Department of Computational Media (and the director of the Center for Computational Experience) at the University of California, Santa Cruz. “There’s a sense of shared reality and accumulated time together, which doesn’t happen in the same way in other media.”

AI sidekick characters come in many forms. Some sidekicks are typically attached to your character, carry your character, or ride around on them—your Yoshis, Kazooies, Cappys, Clanks, and Daxters, who equip the protagonist with more moves and only occasionally operate independently. Some are squadmates or party members who don’t appear on the screen except in battles, cutscenes, certain story sequences, or when executing selected actions, like Mass Effect’s Garrus Vakarian, Destiny’s Ghost, and Halo’s Cortana (who became an IRL AI assistant). Some are largely cute or comedic foils for a (sometimes nonspeaking) avatar, like Wheatley and GLaDOS from Portal, Claptrap from Borderlands, 343 Guilty Spark from Halo, and Dog from Half-Life 2. Others, like Tails in Sonic 2, Luigi in Mario Bros., and Diddy Kong in Donkey Kong Country, debuted as player-controlled characters in co-op mode or merely tagged along after the star in single-player mode, then later got their own games. Animals abound, including Epona from The Legend of Zelda, Trico from The Last Guardian, and Palicos from Monster Hunter World.

I’m not really referring to those sorts of sidekicks. I’m talking about a specific kind that Katharina Emmerich and her colleagues in the Entertainment Computing Group at the University of Duisburg-Essen in Germany have called “companions”—persistently accompanying NPCs that support the player as allies in some nondecorative way. “Companions—out of all NPCs—can have the strongest influence on the player experience and thereby can either greatly increase or impair game enjoyment,” they wrote in a 2018 paper.

Isbister, who devoted a chapter to NPCs in her book Better Game Characters by Design, adds that this type of companion possesses “more social power and autonomy” than “a minion-type character,” but “probably still less of both than the player’s character.” Prominent examples include Ellie in the original Last of Us; Sully and other AI allies in Naughty Dog’s Uncharted franchise; Alyx in Half-Life 2; Quiet in Metal Gear Solid V; Elizabeth in Bioshock Infinite; and Atreus in God of War. Canines count, too: There’s Dogmeat from the Fallout franchise, and the dog from Fable 2. But not the dog from Duck Hunt—that dog is a dick.

“The most important design aspects of a companion are believability and personality,” Emmerich says via email. “Players seem to prefer companion characters who have their own head and objectives (which are not necessarily in line with the player’s opinions or the game’s objective) and an interesting background story. Players want to relate to the companion and build a relationship—which can only happen if the character presents some personal characteristics. … The relationship between player and companion should not be static: Players get to know the character piece by piece, so their relationship evolves and changes over the course of the game and is also influenced by the game events they ‘experience together.’ … Some part of the appeal of companions comes from our desire to share our experiences (both success and failure) with others, because they become more meaningful this way.”

Companions act as goal-givers and instructors, provide the player with cues about how to feel and details about the game-world, and support the suspension of disbelief. They also supply the illusion of a social aspect to a single-player game, which is particularly important in games where the protagonist is supposed to be part of a group as opposed to a solitary explorer. According to Emmerich’s research, players care less about the companion’s utility in gameplay—following specific directions in squad-based shooters such as Republic Commando, autonomously killing enemies like the companions in The Last of Us and its sequel, or delivering supplies like Elizabeth in BioShock Infinite—than they do about believability and personality. Adina Friedman, who created and researched companion characters and more basic bots in shooters while studying at Southwestern University, echoes that finding, reporting, “I’ve been happy to forgive or ignore glitches or weaknesses in characters that I enjoy, and at other times a character can be super strong, but I refuse to take them with me on missions because I don’t like their personality.”

As an only child who grew up gaming alone for long stretches—which is hopefully less sad than it sounds—I’ve always appreciated the presence of an on-screen sidekick. I’m also easily unsettled by scary games, and the inclusion of a convincing companion as an on-screen security blanket seems to make me a bit braver. What’s more, the oft-lamented decline of local (or “couch”) co-op in the era of online multiplayer has preserved the importance of NPC companions for players who gravitate toward single-player campaigns, even if they have access to living, breathing companions who could pick up a controller.

The digital deaths of NPC sidekicks have been devastating gamers dating back to Floyd, a doomed but beloved robot in the 1983 text-adventure game Planetfall. But capable companions can set up some of a game’s most memorable moments even in (virtual) life. Friedman cites sidekicks John Lugo and Alphonso Adams from the 2012 shooter Spec Ops: The Line as her personal favorites.

“Gameplay-wise, they’re completely capable in combat, they can take commands for [whom] to target, but you can also let them go on their own and they’ll do fine,” she says. “More important, though, is their role in the narrative. They often are presenting two different arguments or ends of a spectrum, with the player character mediating or choosing between the two, and they act as outside observers for the player’s actions. There’s a chapter in the game when the player is separated from them and has to fend for themselves for a while before the two of them return. During my first playthrough, I can clearly remember saying, ‘Oh my god, I’m so glad to see you guys’ out loud when they came back.”

I know the feeling: Some of the prized companions I’ve played with are still real to me. But the list of high-quality companions is still fairly limited for a reason. As Emmerich wrote in 2018, “The integration of a companion into a game is challenging and costly.” Martin Cerny of Charles University in Prague experienced that difficulty firsthand when he tried to create a “likeable and competent” AI sidekick, a process he described in a 2015 paper. “When it comes to character behavior,” he concluded, “even the best AI algorithms often do more harm than good unless the AI is perfectly aligned with the overall game design.”

Some sidekicks are merely annoying, like Navi, Link’s fairy companion in the otherwise-wonderful Ocarina of Time. The book Fundamentals of Game Design notes that Navi “offers valuable advice at key points in the game,” which is technically true, but the companion’s constant interruptions and limited sample of sounds made her more maddening than helpful.

In other cases, an NPC sidekick can break boundaries by impeding a player’s progress or overly easing it. “In creating an AI sidekick, a very delicate balance has to be found: If the sidekick is incompetent, the gameplay is frustrating as the player has to babysit the AI,” Cerny wrote. “On the other hand, if the sidekick is too good at helping the player in the game, it diminishes the player’s sense of achievement.”

In a game like Gears of War, which came out in 2006, the ally AI was embarrassingly bumbling.

By Gears of War 3, which was released five years later, the AI had improved—perhaps too much. On normal difficulty, the player could essentially sit back and let the computer overcome enemies on its own.

“A bot that is too good can leave the human player feeling like a spectator in their own story,” says Jacob Schrum, an associate professor of computer science at Southwestern University who has extensively studied AI combat behavior in first-person shooters. “This partly explains why NPCs are sometimes a burden to overcome in certain games. However, the best NPCs will successfully help the player without eliminating the challenge of the game.” Schrum adds that players prefer NPCs to play in a “predictable” fashion, mimicking human behavior and offering evidence of teamwork even if the enhanced immersion makes the computer-controlled allies less effective. In a 2013 paper, Jonathan Tremblay and Clark Verbrugge of McGill University noted that “adaptive companions” that adjust to the gamer’s tendencies run a lower risk of “breaking the player’s experience by not respecting the player’s intentions and gaming style.”

Companions can also break the player’s experience simply by being broken themselves. Trico from The Last Guardian was notoriously tricky to control, and earlier NPC sidekicks presented even more serious obstacles. As Emmerich and her colleagues wrote in 2018, “Most negative experiences reported can be attributed to inapt behavior like blocking the player character’s way, interfering [with] their strategy, or giving advice that is useless or inappropriate. Having to take the companion into consideration all the time proved to be a main complaint and fits in with the general preference for companions who are able to take care of themselves.”

Sidekicks that require the player to escort and protect them—Yorda in Ico, Ashley Graham in Resident Evil 4, Trip in Enslaved: Odyssey to the West—raise the stakes of every combat encounter, although their vulnerability often makes them the most infuriating fake friends. No escort character has caused more broken controllers than Natalya in 1997’s GoldenEye 007. An easily offed staple of “worst sidekick” lists, Natalya invariably wandered into the player’s field of fire, walked into doorframes, or showed little concern about exposing herself to enemy fire.

Companions have grown much more sophisticated since those blocky and block-headed days. Large leaps in visual fidelity and motion-captured facial expressions have made companions appear much more lifelike, although that surface-level realism has put pressure on programmers to keep pace: As Emmerich and her colleagues observed, “the more human-like a NPC is perceived, the higher are the player’s expectations regarding its behavior.” In The Last of Us, the intelligence of Ellie’s AI couldn’t quite match the quality of her writing or the depth of her bond with playable protagonist Joel, which led to some jarring stealth sequences in which Ellie eluded enemies who should have been able to detect her. Companion AI and combat ability were souped up for the sequel and can be manually adjusted in the game’s difficulty settings.

Which brings us back to Jesse, the authentic companion who kicked off this journey. In The Last of Us, Isbister says, “It’s a nuanced, dark set of situations that you find yourself in. I would imagine it’s good to have company in such situations, and also, seeing another person’s reactions and motivations alongside yours as the player character probably also helps to immerse you in the moral complexity of it all. To underscore the difficulty and also heighten the emotional tone of the experience.”

I can testify to that. With apologies to Ellie and Abby, the real treasure of The Last of Us Part II was the computer-controlled friends I made along the way. Almost 35 years after an 8-bit Link left on his first adventure, it’s still dangerous to go alone. When you can, always take a companion.