In Blazing Angels, your allies are invulnerable to your attacks. However, at the end of certain missions, you can see that they have a smoke trail behind your plane, indicating that while being hit with eight air to ground rockets and dozens of thirty millimeter bullets (which kills most enemies in the game in less than a second) isn't enough to damage them visibly, a few machinegun rounds fired off by the enemy is enough to set them on fire.
In the fifth Ace Combat game a superweapon is capable of destroying any plane below 5,000 feet in one hit... except your wingmen, who can be cruising at 2,000 without a care in the world. Almost makes one wonder if they know something you don't.
In 6 you can shoot down allies with your guns. Even named ones, except Shamrock, who mutters something about his plane, and flys back to base.
The second Halo game granted Sergeant Johnson immortality. You could literally keep chucking grenades at him and he would just stare at you, and if you did it for long enough he'd eventually ask what you were doing.
At one point in the first level, Sarge, as he is affectionately known, tells Master Chief "If you get scared, you can hide behind me." A surprisingly viable tactic!
In the second game, many supporting characters, such as Sgt. Stacker, are still killable, but they still come Back from the Dead later.
Same goes for his Covenant counterpart Rtas "Half-Jaw" 'Vadumee. You can keep striking him in the back, which would be an instant-kill for anything else, and although his shield can be seen dissipating he stays alive.
In Halo 3 plot-important allies can be killed, but they'll stand back up in about ten seconds like nothing happened. You can even loot their weapons, including an infinite-ammo SPARTAN laser at one point.
Once, it even happens with a plot event.
Let's not forget Guilty Spark in the first game. Although it carries over to when the AI turns hostile, yet manages to disappear during the boss fight in the final game (though this may be because this time you're attacking him with the super-badass SPARTAN laser).
Apart from Guilty Spark, however, Halo 1 does not have Gameplay Ally Immortality, which makes it rather odd if Johnson dies in one level only to reappear alive and well in another. But extremely annoying if Cpt. Keyes dies, especially since You eventually mercy-kill him when he's mutated into a Flood "proto-Gravemind".
And if you decided to pistol-whip Keyes when you first saw him, he would die and you would be locked on the bridge and attacked by furious, invincible Marines.
The Call of Duty games have this in that characters important to the plot will never die in battle, no matter how often they are hit, but generic allies will fall and be replaced by off-screen reinforcements. If an essential character takes an excessive amount of damage in a short amount of time, they will sometimes fall over and look as if they have died, before getting back up again a few moments later.
If the objective calls to protect said important NPC, then you better, they'll go down like the rest of the generic NPCs.
Immortal teammates can be a lifesaver in some situations, especially on harder difficulties. You simply find a place to hide while your ally stands out in the open and guns down the enemies for you. When they get hit by enemy fire they simply fall over, then get back up and resume fighting.
The whole point of Guardians Crusade is to return a baby monster to its home in God's Tower. Although the aptly-named Baby can die in battle, if you end up winning the battle, Baby will be automatically revived at the end of it. Of course, he'll hate you for letting him die...
In X-Wing: Alliance there are some missions in which certain AI-controlled craft will never go below 1% hull integrity. In the last mission, which puts the player in control of the Millennium Falcon during the attack on the second Death Star, there is an X-Wing escorting you that is exempt from destruction in this manner, presumably because it is piloted by an important SW character. (Likely to be Wedge Antilles.)
In Half-Life 2: Episode 1 & 2 Alyx isn't exactly invincible, but (unlike you) her health regenerates extremely rapidly, making it unlikely for her to die unless you deliberately sit back and let her get mobbed by multiple enemies. Most named allies in the game are the same way, and the game designers try to engineer situations where this isn't actually shown and pull it off for the most part.
The Abbot and Costello pair in Episode 2 can, according to commentary, hold their own against the Antlion horde. You, however, will get your face chewed off.
The 2D Sonic the Hedgehog games where you can play as Sonic and have Tails following you, Tails can get killed numerous times but still come back a short time later. This made boss fights a lot easier if you had a friend playing with you as you could have Tails attack the boss without worrying about taking damage.
Parodied in a later episode where a bunch of Facehuggers attack him and a Chestburster comes from his chest, but Tails doesn't die.
In Kingdom Hearts and its sequel, you allies will get up a few minutes after losing all their health. If you lose all your health, however, it's an instant game over. This is probably justified, since they are Disney characters.
The in-game justification for this is glossed over very quickly, but basically boils down to your partners simply being knocked out, and Sora losing his heart any time he dies. Because the Heartless/Nobodies are very vulnerable to the Keyblade, they don't take any chances.
The occasional NPCs such as Cloud and Leon that help you fight Heartless during events. While they can be hit, they seem to be invulnerable, and will pretty much kill all the enemies for you.
Both played straight and subverted by the Dream Eaters in Kingdom Hearts 3D. While your own Dream Eaters can die off for good by running out of HP and not receiving assistance, the temporary ones you can call to your side using friend portals are completely indestructible and simply get knocked around by attacks.
For a laugh, in the first game's fight against Maleficent's dragon form, you can simply climb the branches to get as high up as you can and let Donald and Goofy do the whole fight for you (useful, as she can be quite hard unless you're diligent about leveling up).
Played painfully straight in Dragon Quest VII, in which the weakest of Guest Star Party Members can potentially soak up more damage than the final boss, although the AI is suspiciously good about hitting you rather than them.
Happens all the time in Dragon Age: Origins and Dragon Age II: If people fighting alongside your party (blue-circled) are important, they will continue to fight, even if their health is depleted.
Uncharted: Drake's Fortune, has several areas where you fight alongside Sully or Elena, and while you die quite easily — often from a single hit — they can pointedly ignore direct hits from grenade-launchers. Too bad they almost never hit the enemy...
Knights of the Old Republic series: No members of your party (even the PC) actually "die" in combat (even after being blown up by grenades and lit on fire), instead merely collapsing in pain. They can be brought back into the battle with the correct healing technique, and they will struggle back to their feet with 1HP once all combat ends, but if all of your party members are knocked out it's game over. One party member even had a chance to get up right after getting "killed" as his special ability.
This was one of the few games where even your character could do this—you only got a game over if your whole party was killed. Of course for many boss fights, the game usually found some way to immobilize your teammates (usually by sith who put your friends in stasis, why only your two friends, and not all three is unexplained). Judging by the later lightsaber sound effects during the game over black out screen, it's implied that if you lose, whoever you were fighting will go on to kill your immobilized teammates.
You can use your party members to clear mines...
The same goes for Neverwinter Nights 2 and its first expansion (it is, however, not true for the second expansion, where dead party members would stay dead unless resurrected.)
When you summon your allies in Drakengard, they cannot die. The life bar becomes a timer indicating how long they have to run around and kill crap, but they technically cannot die. If they're struck, it reduces the amount of time they're summoned.
In the sequel, the hero and his allies all have separate health meters. Now, if your onscreen character dies, you get booted straight to the game over screen.
In The Elder Scrolls IV: Oblivion, plot-important NPCs don't die- they only "fall unconscious". Due to the nature of the game, this means that you can use your unkillable allies to grind your stats to ludicrous levels.
Occasionally, a plot-important character will follow you around for the duration of their quest and can effectively be used as an invincible escort as a result; the "Followers" page on the Oblivion wiki notes who is plot-important ("essential"), and even provides basic instructions on how to load up your followers with improved gear, so that you can have the Emperorat your back with incredible glowing equipment.
In Arena and Daggerfall, the allies important to the storyline were all sprites whose interaction was limited to talking to them — they couldn't die, but they were of no help, either.
In fact, since Shadowmere, the Dark Brotherhood "gift" horse, is considered a plot-important NPC one could knock her unconscious and then store items inside of her "dead" body. A very useful tool for characters with low strength.
Age of Mythology plays this game. Story PCs will be able to rise again if allies are around and enemies are not. Could be justified by Athena watching your crusade. One character notably has to block Arkantos's team from getting to him to make a Heroic Sacrifice.
Age of Empires III has its heroes operate similarly but they at least lampshade it by by complaining about their pain in a borderline plea for euthanasia ("Every moment I live is agony.")
Kirby's Fun Pack (Kirby Super Star in the states) had partners which could be controlled by a second player. While they have the same amount of health as Kirby himself (which makes them much tougher than most of the enemies with the same form, though), they can't die from falling into bottomless pits or being crushed by scrolling levels (they just warp to Kirby) and if they lose all health, they can still be turned back into the ability and be reformed instantly. Or they can run into an enemy with an ability and get turned into the respective partner with full health. Finally, they can share health items with Kirby.
Super Smash Bros. Brawl partially follows this trope during the Subspace Emissary in co-op. Player 2 can fall off the stage and be put back on it, but if an enemy attacks the player and they get pushed too far off screen, it counts as a KO. When you get caught in an non escapable battle, falling off the stage still counts as a KO for player 2.
In Paper Mario, Mario's partners have no health meters; if they are struck by enemy attacks, they are merely incapacitated for a few turns. The same is not true in the sequels.
In Mickey Mousecapade, the player controls Mickey and Minnie simultaneously, and Minnie never takes damage. Separating them in a few situations (like a boss fight with Pete) and attacking with Minnie makes dying impossible.
Somewhat subverted in SoF 2, where although friendly soldiers can't be killed by enemies, they CAN be killed by friendly fire, and if that happens, you spontaneously die as well.
This also ends up inverting a typical escort mission in Jedi Academy, where the so-called escortee is Kyle Katarn, who can pretty much take care of everything in the level for you.
Averted in TimeSplitters: Future Perfect; all of Cortez's sidekicks seem to be immune to gunfire, but will in fact eventually drop dead after taking too many hits from anything - even you. There are also areas where a sidekick will suddenly sprout a health bar, forcing you to watch their back while they perform a task while suddenly becoming unfathomably weaker.
The Legend of Zelda: A Link to the Past included a few escort quests, most notably one near the beginning of the game where you must take Princess Zelda through a variety of dangerous environments to the Sanctuary. Fortunately, she's invincible, with monsters passing right through her - her only apparent sign of corporeal existence is her assistance with pushing a bookcase.
The Legend of Zelda: Link's Awakening has a couple of characters who follow you for a while, including love interest Marin. Not only do all attacks pass through her, she can somehow keep up with your Pegasus Boots, make jumps you need the Roc's Feather for, and (if you do a little Sequence Breaking) walk on water when you're swimming.
In The Legend of Zelda: Spirit Tracks, Zelda gains the ability to control phantoms, which are giant armored knights. She can't be killed, but some attacks will stun her, she'll panic from the sight of rats, and the final boss can take control of her and make her attack you.
All important allied characters in Deus Ex are unkillable. A lot of the generic UNATCO troopers in the early levels are immortal as well, though only in areas without enemy presence. However, the game also had a few memorable subversions: among all the invincible UNATCO soldiers after the first mission is a single vulnerable one, and killing him gets you some extra dialogue with Manderley. Much more importantly, you can kill Anna during the confrontation with Lebedev.
The sequel, InvisibleWar had several immortal NPCs as well, but it also de-emphasized the issue by segregating the player from them. A lot of conversations with plot-relevant characters took place over video screens and across unbreakable walls.
There was also areas where all your weapons are deactivated, thereby preventing you from attacking plot important characters prematurely. However, even if you glitch your way through and have your weapons still active, everybody in the area is still invincible anyway.
Eileen in Silent Hill 4 can't die, but she does take damage, and that will affect whether she survives the final battle or not.
In Mass Effect, all the characters are subject to this, except for Commander Shepard (the player character, whose death results in Game Over). This sometimes becomes rather silly, as NPC's cannot be directly shot, (Except with the spray-bullet shotgun) and never react to weapons fire in any way. Even detonating a grenade an inch in front of your direct superiour merits not a flicker of an eyebrow.
In Fire Emblem, where death is permanent, most allies not under the players control will simply be injured and leave the fight if they run out of health instead of dying. Radiant Dawn has a level with Kurth, who is believed at this point to be a civilian with no combat skills. He does, however, have much more health and defense than any of the player's characters. This means he can be used as an effective guard (albeit one that doesn't fight back).
Some guest characters in Final Fantasy Tactics A2 simply can't be killed, just reduced to 1 hitpoint. They usually stop attacking in this case unless you heal them, but they can still be attacked by enemies.
Enemies will actually prioritize guest characters that are still able to fight but can't be reduced below 1 HP, so an entirely valid tactic is to use a potion or healing spell on those characters to restore the bare minimum HP, and then letting the enemy attack them. It only works if they are within the enemy's attack range, and when they are reduced to 1 HP, the AI starts ignoring them again, making for a delicate balancing act.
In Star Wars: Republic Commando, your NPC allies weren't any more immortal than you. Interestingly subverted though, in that it wasn't an instant game over if you the player died, only when all 4 members were dead. Just as you could revive them if they dropped, the NPC's could revive you if they survived the firefight, or if you gave the order to break cover and revive you on the spot. Of course, none of the squad is quite dead at that point, generally groaning piteously and trying to stand; when it happens to you, you can look around and order a squadmate over. Enemies just ignored you, and you couldn't shoot.
Curly in Cave Story, during the last part of the Labyrinth. It especially helps that she wields the Machine Gun (or Polar Star if you swapped weapons with her when you were given the chance) and can take out enemies for you.
This trope is a plot point in the videogame-based webcomic Kid Radd.
In Final Fantasy Adventure, your character has various temporary allies who provide backup or just follow him around. They're all immune to damage. The Video Game Remake, Sword of Mana, changed this — and fortunately thought to provide some AI control in case you don't want your ally Leeroy Jenkinsing himself to death all the time. However, this didn't remove the silly factor so much as move it: when an ally dies, he or she follows you around as a ghost, and is instantly revived if you touch a health-refill statue.
Pets and guests in the Flash-based RPG AdventureQuest can't be targeted by enemy attacks. As a result, any ally that can heal your character is extremely helpful.
Hand Waved in the Dragon Fable Design Notes, which say that NPCs taste funny. Though DF does have occasional boss monsters that will attack your allies, they're few and far between.
A feature of the original Fable was that you could kill pretty much any NPC or quest-giver. Of course, because the game has to find a way to carry on regardless, no-one is too bothered if you've beheaded your five previous employers.
Fable II has consequences for murder, including the town economy slumping, housing prices falling, and everyone in town becoming afraid of you. Then you fart a couple of times and they think you're hilarious again. If they don't run away before you finish the fart.
Fable III actually allows you to turn ally immortality on and off yourself by way of a D-pad option that comes up when you draw your weapon. You can't hurt friendly NPC's unless you turn off the "safety".
This also appeared in Suikoden Tactics: plot-vital characters never have to worry about Final Death; no matter how much punishment they take, they'll simply retreat. Everyone else, however, has a chance of dying on the battlefield — with the only real forewarning you get how their 'defeated' quote changes to Famous Last Words. However, not everyone you'd expect to have this immortality actually receives it...
Hewie, the cute dog in Haunting Ground, can't truly die in Normal mode, unless you Kick the Dog a little too much, causing a cutscene death later in the game and giving you the worst possible ending. He can be knocked out, but Fiona can massage and pet him until he wakes up. In Hard mode, if Hewie gets knocked out, he dies for good and the ''Acta est Fabula'' will greet you presently.
In Infinite Undiscovery all friendly characters except your main party are invulnerable to all damage, except during one Escort Mission. It's possible to win some boss battles simply by staying away and waiting for your invincible allies to beat it.
Sacred features several unkillable escorts, most notably Wilbur (who joins you fairly early in the first chapter). You can swap out their weapons and/or shields (but not, strangely, their armor) so that they can get kills faster, but you generally don't have to worry too much about them. If one of these NPCs gets 'killed', they just lie on the ground with stars spinning over their heads for about a minute, then get up with about 25% of their hit points restored. It's important to note that only NPCs relevant to the main plot have Plot Armor — anyone you have to watch on an Escort Mission can die, and likely will. I've had to reload many games when someone I had to babysit ran up to a demon (or whatever) and just stood there saying "Take my picture" while it chewed their nipples off.
The Breath of Winter expansion pack for the original Spellforce featured, in the early stages, Grim and Lena, who would follow you around everywhere and never, never die, even if surrounded by a dozen melee-focused opponents. Although, unusually for this trope, they began limping and ended up pretty much useless if their health bars went into the red, but a quick healing spell would get them back up to continue cruising through their your opponents.
In Clive Barker's Jericho, the game ends if every member of the Jericho Squad dies. However, if a few of them have died during battle but the player manages to finish the battle, the incapacitaed squad members usually get right back up again, even without the intervention of Ross' or Rawlings' healing powers.
Fallout 3. After you find your Dad, he sets off to Rivet City, which is about half a map away. On foot. If you decide to take a walk with him, you get to watch him take the highway into D.C., right through everything from raiders carrying missile launchers to super mutant masters with miniguns, wearing nothing but his jumpsuit - and "become unconscious" over and over again... just to get up half a minute later and continue on his merry way. And to make the trope even more fitting, all this so later the plot can kill him anyway.
In fact, any plot-sensitive NPC has this ability. Try firing the sniper rifle point blank into the back of Elder Lyons' head. He dies instantly and stays dead for a few seconds, then gets up, unfazed. Unload an automatic rifle clip into Dr. Li's face some time for cheap giggles; not only does it only take off about a bar of her health, she's so unfazed that she doesn't even go hostile.
In addition to Dr. Li, important characters include all children, Jonas while you are 10 or 16 years old - further following the example of being invincible until he needs to die, and one Robobrain in Vault 112 - which has exactly two lines of dialogue.
That Robobrain is vital to the main plot, since without it you wouldn't get into the Tranquility Lane simulation to rescue your dad.
And there must be at least one doctor, one repairman and one trader living in the Wastes so the player ins't left stranded. There just so happen to be one of each character with invincibility.
The PC version makes it rather easy to grant and take away this immortality to/from NPCs and items as well by use of console commands.
Fallout 1 and 2 totally averted this, though. If you are so inclined, you can kill every single NPC and still win. (Apart from the residents of Arroyo ? killing them is possible, too, but this ends the game.) The sole exception in Fallout 1 is, of course, the Vault 13 Overseer, until it's... his time.
In Guild Wars, the Escort Missions of the original campaigns, despite the ease of resurrection for party members, would automatically be failed if your ward or ally was killed. However, since the Eye of the North included gigantic dungeons that are almost impossible to complete without a few party wipes, allied NPCs would automatically resurrect themselves when there were no monsters in the area, and apparently without any death penalty (game mechanic that temporarily reduced stats for dying).
Seen in Gears of War, and featured via legacy code in the vast majority of Unreal Engine 3 games based off the Gears of War game code (i.e. Turok, Blacksite: Area 51, Haze, etc). If one of your NPC squadmates is severely injured in a firefight, instead of dying, they'll simply drop to their knees in pain. They get back up again good as new once all nearby enemies are dead, or if you simply walk up to them and give them a pat on the back.
First seen in Rainbow Six: Vegas and Star Wars: Republic Commando. Unreal Engine games really seem to love recycling this particular bit of code.
In Vegas, it is possible for teammates to die instantly if they are caught in the enemies'Instant Death Radius. Very easy to notice this in terrorist hunts in Vegas 2 by telling your team to stack up at a door and then triggering the enemy spawn. As soon as they come through the door with guns blazing, Game Over.
In Legend of Legaia, Noa has very low stats in the beginning of the game, but is joined by the wolf Terra, who cannot be killed. It is possible to take advantage of easy level grinding during this time.
Both subverted and played straight throughout the Wing Commander series, depending on the specifics. In some games and missions, NPCs can die (complete with funeral scene), while in others they eject to be available the very next mission in others... or at least remain available until they're doomed by the plot.
Happens in two ways in the Might and Magic series. Although you can kill any NPC walking around town, all the quest givers and important story characters are in houses or other buildings where you cannot attack. Second, you can hire NPCs to travel with you and give you buffs, but no matter how much of a beating you take, they are never harmed.
Lampshaded in Battlefield: Bad Company. Your NPC squadmates will say things like "Ow, my face! Can't believe I made it!" when they are hit.
In The Godfather: The Game, plot-important characters like Sonny or Monk will not take damage in battle. Sure, they'll bleed when shot, but if you look at their health bar you'll notice that it doesn't shift one bit.
Your NPC allies in Time Crisis can't die, either from enemy fire or you shooting them, but you do take a point penalty for shooting them (-1,000 for Captain Rush in Time Crisis 4, -5,000 for Christy in II and Alicia in 3.)
In Valkyria Chronicles, losing Welkin (and sometimes Alicia) is a game over, and regular recruitable characters are subject to Perma Death if they're captured by the enemy or left for dead on the field, but then you have the plot-critical playable characters (Rosie, Largo, Zaka, and Alicia when her death isn't an instant-lose condition), who will merely "retreat" under the same conditions that an ordinary recruit would die.
The major ally in BioShock 2 is of the same enemy type as some of the Demonic Spiders, with the exception of being invincible outside of exploits and glitches.
BioShock Infinite features this when you first talk to the Lutece Twins. Not only will shooting a pistol into their faces have no effect, but they even mock you for doing so. Later, you find out that there's a good reason that shooting them has no effect.
Each line is after you try to shoot one of them:
Rosalind: "Missed again."
Robert: "You really shouldn't squander your ammunition or your friends when you have little of either."
Rosalind: "Four out of five?"
Rosalind: "And another miss."
This is played straight with Elizabeth, however - during combat she is invisible to (and invincible against) all enemies.
This is justified, as troops loyal to Comstock think Elizabeth's their messiah. The Vox Populi may just be prioritising hazards.
In the video game for Transformers: Revenge of the Fallen in most escort missions the escortee dies when hit with enough fire. However, when you escort Ironhide (one of the only mission where you actually have NPC allies) he will take damage to a certain point and then will become effectively immortal.
In Left 4 Dead 2, the old survivors in The Passing's finale are completely immune to damage. Shooting them does nothing. It is strange that they are invincible, considering that the zombies outright ignore them and target only your group.
In Freedom Fighters the fellow guerrilla fighters that the main character (Chris Stone) can recruit are essentially invincible. If they are downed by enemy fire, they only require a single medpack a piece to get them back into the action, and they're practically immune to friendly fire.
In Crysis: Warhead, there are a couple sections where you have a friendly team of U.S. Nanosuit Soldiers escorting you through an area. Although they can be knocked down if they take enough damage, they'll get back up again after a couple dozen seconds no worse for wear. Possibility due to a glitch, there's a very specific circumstance where they can suffer Final Death if they get grabbed and thrown by the giant alien robot boss. They all reappear next to you good as new once you transition to the next map, though.
Perfect Dark has a small, but surprisingly satisfying exploit in the HQ/hub/training level. The hanger has a box that you can grab. Naturally, you can take this anywhere, including the firing range where the tests for all the weapons take place. This room is separated from a smaller area where scientists are watching you testing the weapons. Usually the door separating you from them can't be opened during a test and you obviously can't take weapons out with you. However, you can position the aforementioned box in front of where the door slides allows you to shoot any of the weapons at the scientists outside. Aside from blood stains and arrows sticking out of their body (and the lights blown out if you used explosives) they show absolutely no sign that they were ever hurt.
However, one of them will occasionally say "Just... leave me alone, would you?"
In pretty much every Ultima game, while your allies could get killed as easy as anything, a quick trip to Lord British (or someone else filling the role) is all that's needed to get them back on their feet. The main character gets the same benefit - even saving the effort of hauling back the corpses. Death Is a Slap on the Wrist, indeed.
A few missions in Jedi Academy have the player fight alongside legendary Badass Kyle Katarn. As you might imagine, he's insanely difficult to kill, as the game gave him superhuman reflexes, near full Force power, and more than double the HP of the Final Boss. Mooks can't kill him; the only way he can ever die is if you're unfortunate enough to encounter a Game-Breaker Glitch which causes Kyle to accidentally treat you as an enemy. (This has one of two outcomes: You die, or you manage to kill him after an insane amount of effort, in which you are rewarded for your effort by failing the mission for letting your ally die.)
Ciel in the Mega Man Zero series, whenever she appears in an Escort Mission. When she gets hit, she still reacts as if taking damage, but she never dies no matter how many times she gets hit. Letting her get hit only reduces your score for that mission.
On your first playthrough of the game you will spend much time and effort babysitting Ashley to prevent her from being hacked to death by mutants or being grabbed and carted off by mad cultists. However, an unlockable costume dresses her in a full suit of plate mail. This makes her completely immune to damage and also makes her so heavy that enemies cannot pick her up to abduct her.
In Gothic, two people in the Old Camp can escort you to the New Camp and the Swamp respectively. While the paths are full of beasts that make lone travel deadly, they are tough enough to kill everything... and give you the experience points. It's actually worthwhile to draw the attention of beasts and let your friend kill them for you.
Warhammer: Dark Omen and Shadow of the Horned Rat, had certain warriors with important dialogue join your warband to help advance the plot. Not only could they not be killed permanently, they also costed NO MONEY!!!!! which almost guaranteed that you stick them in a fight. They aren't immune to damage but even if they got slaughtered in a battle, they'd be up in perfect shape for any upcoming battle so long as you win - they didn't even spend any time on the injured roster. So feel free to charge that lone Elf ranger into a ravening horde of Black Orcs or a Skaven Warpfire Thrower team.
Diablo III has Leah, Decard Cain, Tyrael, and Adria occasionally join in the fight against evil directly, accompanying the human players and their NPC companions on their quest and taking the fight directly to the enemy. To avoid being a slow-paced Escort Mission, this trope is applied. This also happens when the player first meets each of the Companions throughout the story; the HUD distinguishes invulnerable characters by simply not giving them a health bar display.
Named NPC allies in Freelancer are generally immortal in missions where they appear. They'll take damage down to roughly about 10%, at which point the game no longer counts damage against them. Unfortunately, seeing as how this is a space combat simulator, you can't actually use them as cover, but they can clean up enemies while you stay out of the thick of it. If it's a Plotline Death, though, all bets are off.