Fallout: New Vegas companions

Permanent companions
It is possible to have up to two legitimately acquired companions in the party (one humanoid and one non-humanoid). All permanent companions have a unique quest line that can be followed in order to "upgrade" them; usually giving them new armor or new perks.
 * Arcade Israel Gannon, a member of the Followers of the Apocalypse in 2281 who can be found in Freeside's Old Mormon Fort.
 * Craig Boone, a former NCR 1st Recon sniper with a grudge against Caesar's Legion and a chip on his shoulder. He can be found at night in the dinosaur at Novac. Can be recruited after completing One for My Baby and choosing particular dialogue.
 * Lily Bowen, a friendly nightkin and former assassin in the Master's army. She can be recruited in Jacobstown after gaining the quest Guess Who I Saw Today.
 * Raul Tejada, a ghoul mechanic and former gunslinger held hostage by Tabitha, the mad leader of the State of Utobitha. Can be found in Black Mountain.
 * Rose of Sharon Cassidy (also known as Cass), a former caravan merchant and the daughter of John Cassidy. She can be found in Mojave Outpost.
 * Veronica Santangelo, a disillusioned Brotherhood Scribe. She can be found at the 188 trading post.

Non-Humanoid companions

 * ED-E, a prototype eyebot designed for combat and reconnaissance operations. It can be found in Primm, in a non functional state, lying on the counter inside the Mojave Express, which is labeled in-game as "Nash Residence." ED-E requires 65 Repair (or 35 Repair if possessing a 55 Science skill) to fix it, or 3 pieces of scrap metal, 2 sensor modules, and some scrap electronics.
 * Rex, is a cyberdog in Freeside under the ownership of the King. He is suffering from neural degradation as his old brain decays. He can be recruited after finishing Nothin' But a Hound Dog.

Dead Money companions
These companions are only available while playing the Dead Money add-on, they are only temporary and will not follow the Courier around as standard companions do:
 * Dean Domino, a ghoulified pre-War lounge singer. He is found in the Residential District of the Sierra Madre.
 * Christine Royce, a mute ex-Brotherhood scribe found in a malfunctioning Auto-Doc at the Sierra Madre medical district.
 * Dog/God, a nightkin found in the Villa Police Station of the Sierra Madre.

Honest Hearts companions
These companions are only available while playing the Honest Hearts add-on, they are only temporary and will not follow the Courier around as standard companions do:
 * Follows-Chalk, a novice scout for the Dead Horses tribe with wanderlust. He first accompanies the Courier for a while, before permanently returning to the Dead Horse's camp after a certain level of progress has been made.
 * Waking Cloud, the midwife of the Sorrows tribe worried about her husband. She accompanies the Courier upon Daniel's recommendation.
 * Joshua Graham, former Legate and reborn New Canaanite acting as war chief for the Dead Horses. He will only accompany the Courier either during the Flight from Zion final quest or the Crush the White Legs final quest.

Old World Blues companions

 * Roxie, a cyberdog which can be created using the available splicing machine and after acquiring the correct holotape needed. Roxie will follow around her creator throughout the X-8 research center only; she cannot leave it. When interacted with, it is only through dialogue rather than the command wheel. Unlike Rex, it is not possible to open her inventory to give/take things. Even in non-hardcore mode, she can die, but can then be re-created an infinite number of times.

Lonesome Road companions

 * ED-E, a cloned version of ED-E, armed with a modified arc welder. This version of ED-E cannot follow the Courier to the Mojave Wasteland and is always classified as essential, even in hardcore mode.

Temporary followers
Temporary followers are NPCs that can accompany the player character during quests, or they might even offer their support only for a limited time or within a certain area. Unlike permanent companions, temporary companions do not use the companion wheel, do not heal automatically, and with the exception of deputy Beagle, do not have an option to exchange equipment.
 * Bodyguard (hired in Freeside)
 * Kings (hired in Freeside)
 * Orris (hired during G.I. Blues.)
 * Cannibal Johnson (Enclave Remnant)
 * Deputy Beagle (during My Kind of Town)
 * Logan (during part of Wheel of Fortune)
 * NCR Ranger (activated via NCR emergency radio)
 * NCR trooper (activated via NCR emergency radio)
 * Private Halford (in Camp Guardian caves)
 * Rhonda (during part of Crazy, Crazy, Crazy)
 * Securitron Mk II (or securitron Mk I) (during No Gods, No Masters or All or Nothing)
 * Ted Gunderson (during Beyond the Beef)
 * Ulysses - the original Courier Six and significant character in Lonesome Road. Depending on the dialogue chosen, he will either become a temporary companion, or will become hostile.
 * Lost Mountain bighorner calf


 * Ulysses was also planned as a permanent companion, but was cut from base game.
 * Betsy the Brahmin was planned as a companion, but was cut from the game.

Nerve
Each companion also has a special attribute referred to in the game as Nerve. Nerve boosts the damage and Damage Threshold of each companion by 5% for each point of Charisma a character has. So Nerve grants a total bonus of +50% damage and damage threshold to companions with 10 Charisma and only +5% to both at 1 Charisma. A player character with a starting Charisma of 1 can boost their companion's nerve to the maximum by use of an Empathy Synthesizer, Intense Training, Party Time Mentats (+5), high Survival (so that alcohol adds +3 Charisma), or stacking moonshine with another type of alcohol.

Because Nerve only affects DT and damage, companion quests that bestow other bonuses upon completion (such as movement speed) are potentially less effective for those with high Charisma.
 * Rose of Sharon Cassidy
 * Hand of Vengeance (15% more damage with Guns)
 * Calm Heart (+50 Hit Points)
 * Veronica Santangelo
 * Causeless Rebel (Increases Veronica's Unarmed attack rate by 30%)
 * Bonds of Steel (+4 DT)
 * Elijah's Last Words (+150% melee attack speed, 25% chance to knock down enemies)
 * Raul Tejada
 * Old Vaquero (+33% faster fire rate with revolvers and lever action firearms)
 * Lily Bowen
 * Off the Meds (Permanent stat bonuses, psychotic break at 50% health)
 * ED-E
 * Brotherhood Examination (+8 DT)
 * Followers Examination (+5 DAM, +8 DPS, +0.5% Crit Chance and +15 Crit Damage)
 * Rex
 * Rey's brain (+25 damage)
 * Lupa's brain (+10 DT)
 * Violetta's brain (+50% movement speed)

Weapons
All permanent companions' default weapons have unlimited ammunition. If equipping followers with a better weapon, (DPS is checked) add ammunition and check its count from time to time - they seem to hold on to the new weapon when out of ammunition or switch to melee instead.
 * Companions can use unique weapons with already built-in upgrades, such as the Ratslayer.
 * Most companions use a weapon that's named after them. They share the same main characteristics as the standard version of the weapon and only differ in name.
 * Arcade Gannon's Ripper
 * Arcade Gannon's Plasma Defender
 * Raul's magnum revolver
 * Raul's lead pipe
 * Veronica's power fist
 * Veronica's 10mm pistol
 * Boone's machete
 * Boone's scoped hunting rifle (unique)
 * Cass's caravan shotgun
 * Cass's combat knife
 * Lily's assault carbine
 * Lily's gauntlet (unique)
 * Lily's Vertibird blade (unique)

Armor
Permanent humanoid companions can wear and/or carry all types of clothing that is not associated with any faction. They can also wear all types of armor and power armor that is not related to a faction, even without having the Power Armor Training perk. They only change gear if they are given gear that has a better Damage Threshold than their own outfits. This may not always hold true, however; giving multiple copies of the same outfit to a companion may result in them equipping an inferior copy, despite the difference in DT.

Companions will refuse to wear or even carry faction-specific apparel, except apparel corresponding to their own faction. Giving invalid faction apparel to a companion will result in that companion immediately dropping the item (see Bugs for errors that occur with this); it is not returned, but must be picked up from the ground. This also applies to non-human companions who cannot wear armor, i.e. Lily Bowen, Rex, and ED-E. Only Boone and Veronica are able to carry or wear faction apparel. As a former member of NCR 1st Recon, Craig Boone will carry or wear any NCR faction apparel. Veronica, as a member of the Brotherhood of Steel, will wear Brotherhood power armor. She will also accept apparel specific to the White Glove Society, and giving her the items White Glove Society attire or formal wear, will result in her teaching the unarmed move Scribe Counter. She will not wear these items, however, because their DT is inferior to her default clothing.

Behind the scenes
Several remaining resources point to the game having at one point allowing the ability to have multiple humanoid companions. Primarily, this is found in disabled dialogue options, such as a conversation with Mortimer, where the Courier would have needed to specify which of their human companions they were offering to Mortimer.

Bugs

 * When entering the casinos on the Strip, they will take all companions' weapons. Exiting will not return their weapons they own.
 * On the Playstation 3, this can be fixed by telling the companions to use melee or ranged and switch back to what they were using.
 * Arcade Gannon will sometimes not show the recruit dialogue when you have a non humanoid companion (Rex or ED-E). This seems to happen when Arcade is in the Lucky 38. The only way to fix this is to dismiss all companions, then recruit Arcade.
 * After dismissing Arcade to the Lucky 38 and re-recruiting Boone at the same location, Boone's default weapons shifted to those of Arcade's (Plasma defender & Ripper).
 * Companions who are poisoned will have the full effect of their poison "dumped" when using a stimpak on them. This can result in negative HP which cannot be cured with more stimpaks, and they will die as soon as you exit the companion wheel. (For weaker poisons, this may lower their HP, but since it doesn't go below zero, they don't die.) On the other hand, they will not use antivenom, so the poison may end up killing them naturally. This is especially a problem with Veronica, as she prefers unarmed combat. Patched in Fallout: New Vegas patch 1.3.0.452.
 * A way around this is to turn hardcore mode off (if it is active). The poison will cause your companion to go unconscious a few times, but the poison will eventually wear off. After that, hardcore mode can be turned back on. The reason for this working is simply because in casual mode, companions are set as essential and cannot die.
 * Note: Turning off hardcore mode at any point in the game will disable the hardcore achievement, so this solution is not recommended for anybody who is trying to get that achievement.
 * Yet another way is to tell the companion to wait, which appears to replenish the companion's health and remove the poisoning. The follower can then be asked to rejoin. This can be performed even during combat.
 * You may need to completely dismiss the follower from your party. After reacquiring them later on, if their health has not been replenished, have them engage in combat with an enemy (preferably weak and non-poisonous, to be safe). This should "reset" their statistics, and upon completing the combat encounter, they should auto-heal just as normal. (This may only apply to hardcore mode.)
 * Temporary solution is sending your companion home or to the Lucky 38 then recruiting them again.
 * Alternatively treating them with stimpaks very quickly after they are poisoned will prevent their health becoming negative and actually heal them instead of killing them. Note that it can take 20+ stimpaks to fully heal Rex.
 * The option to dismiss companions to the Lucky 38 will disappear even before Mr. House becomes hostile to the player. This can be fixed by disconnecting from the internet, then removing the latest patch and reloading the game while still disconnected from the internet. Installing the patch again will not allow you to send your followers to the Lucky 38.
 * Items in companion inventories will sometimes disappear from the inventory, yet the weight of the item(s) will still remain.
 * Sometimes, when entering combat from a disance with a melee enemy while your companions are also set to melee and placed right behind you can result in them getting stuck on the Courier and ultimately killing him/her when they try to attack the incoming enemy.
 * If a companion is given a weapon, and has equipped it, accssessing the companion wheel to the option "Use Melee/Ranged" will delete the weapon from their inventory. When switching back the companion will go back to their unique ranged/ melee weapon. Highly suggest not doing this if companion is given a unique weapon.
 * In hardcore mode, when a companion is nearing their carry limit, the player can still place ammo, (any under 1lb), in their inventory even if it exceeds the carry limit. When the player takes ammo out, unless it frees up the carry limit, the player cannot keep placing more ammo in.

Fixed bugs (Patch 1.2.0.31x)

 * If you have several companions in the Lucky 38 at one time and leave weapons lying around, they may start to fight each other, and will die due to the fact that they are no longer listed as an essential. A solution is recruiting the one that the other companions are ganging up on, so that they are essential. Once that companion is knocked out the fighting stops. (Sandbox-AI caused human companions to attack non-humans if the latter carried consumable goods)
 * This may not be completely fixed in player/1.20.352: There was an instance of Rex and/or EDE attacking the human companions upon returning to The Lucky 38: This only seems to have been corrected after going back a couple saves and removing all remaining inventory from Rex; see more details at Rex attacking companions and NPCs.
 * Dismissing companions to the Lucky 38 can sometimes result in them staying in the same location instead of moving to the Lucky 38. So if you cant find your companions at the 38, check where you dismissed them. (As of Patch 1.2.0.31x companions no longer walk to their residence but are teleported immediately).
 * The achievement/trophy The Whole Gang's Here doesn't count the companions you've currently recruited, but rather the total number of times you've recruited a companion. Furthermore, this count is not restricted to a single playthrough, so it will count "repeat" recruiting of a single companion. It is thus possible to recruit the same four companions over two playthroughs and get the achievement.
 * Sometimes when entering a casino, your companion will lose their weapons, and upon leaving, they will still not have their weapons. Mysteriously, the Courier will have it. It is impossible to give it back. The guns run on MCA ammunition, so they just clog up the inventory. To fix this, tell your companion to wait outside. This can happen to ED-E.
 * When entering Lucky 38, Vault 21 or the Gomorrah courtyard with Veronica, she occasionally is removed from the game.

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