Fallout: New Vegas companions

Permanent companions
You can have up to two companions, one humanoid and one non-humanoid. All permanent companions have a unique quest line that you can follow in order to "upgrade" them.

Humanoid companions

 * Craig Boone, an Ex-NCR sniper with a grudge against Caesar's Legion and a chip on his shoulder. He can be found in Novac.
 * Lily Bowen, a friendly nightkin and former member of the Master's army. She can be found in Jacobstown after you get the quest Guess Who I Saw Today.
 * Rose of Sharon Cassidy, also known as Cass, a caravan merchant and the daughter of John Cassidy. She can be found in Mojave Outpost.
 * Arcade Gannon, a member of the Followers of the Apocalypse with a mysterious past. He can be found in the Old Mormon Fort.
 * Veronica Santangelo, a disillusioned Brotherhood Scribe. She can be found at 188 Trading Post.
 * Raul Tejada, a ghoul mechanic and former gunslinger held hostage by Tabitha, the mad leader of the State of Utobitha. Raul can be found in Black Mountain.

Non-Humanoid companions

 * ED-E a prototype eyebot designed for combat and reconnaissance operations. It can be found in Primm.
 * Rex, a cyberdog belonging to The King. It can be found in The King's School of Impersonation in Freeside.

Dead Money companions
These companions are only available while playing the DLC
 * Dean Domino ghoulified Pre-war lounge singer
 * Christine a mute ex-Brotherhood Scribe
 * Dog/God a schizophrenic nightkin

Cut companions

 * Ulysses, Caesar's Legion member

Temporary followers
Temporary followers are accompanying you during a quest or offer their support only for a limited time or area.
 * Sunny Smiles
 * Ted Gunderson
 * Deputy Beagle
 * Securitron Mk II (Or Securitron Mk I)
 * Rhonda, a Mr. Handy Bot looking for Tabitha.
 * Private Halford Camp Guardian Cave
 * Victor
 * NCR Ranger (activated via NCR Emergency Radio)
 * NCR Trooper (activated via NCR Emergency Radio)
 * Body guard (hired in Freeside)
 * Orris (hired in freeside)
 * Cannibal Johnson (Enclave Remnant)
 * Joana (During Bye Bye Love)

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 a perfect charisma of 10 and only +5% to both at the minimum score of 1. A person with a starting charisma of 1 can boost their companions nerve to the maximum by use of an Empathy Synthesizer or Intense Training, Party Time Mentats (+5) and high Survival (so that alcohol adds +3 charisma).

This is most effective with companions that gain increases to their damage or damage threshold upon completion of their companion quests and less important for companions who gain items or abilities not associated with nerve.


 * Rose of Sharon Cassidy - Hand of Vengeance
 * Veronica Santangelo - Bonds of Steel
 * ED-E - Brotherhood Examination (DT+8) or Followers Examination (+??% Damage).
 * Rex - Rey's brain - Violetta's brain - Lupa's brain

Weapons
All permanent companions' default weapons have unlimited ammo. If you equip your follower 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 ammo or switch to melee instead.
 * Companions cannot use weapon upgrades (mods), the upgrades are simply "removed" when you give a modded weapon to a follower and the weapon works in its basic way.
 * Companions can use unique weapons with already built-in upgrades like the ratslayer.
 * Companions can use all types of ammo, but they seem to hold on to the particular type when you equip them with different sorts of one caliber. When out of ammo, instead of switching to the other type of that caliber they use melee instead or sometimes get stuck in a "reload loop". To avoid this, simply give them only one sort of the same caliber.

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 Power Armor Training. They only change gear if the given one has a better Damage Threshold than their own outfit.

Faction armor
Companions will refuse to wear or even carry faction-specific apparel, except apparel corresponding to their preferred faction (see Exceptions). Giving invalid faction apparel to a companion will result in that Companion immediately dropping the item (see bugs); it is not returned to the player, 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.

Exceptions

 * Some Companions can be considered to be already allied with or accepting of certain factions, and as such will carry and/or wear apparel specific to those factions. Although these Companions will wear some faction-aligned apparel they are not subject to the same effects, namely they are not treated as being disguised as a member of the faction in question.


 * Boone
 * As a former member NCR 1st Recon, Craig Boone will carry or wear any NCR-faction apparel.


 * Veronica
 * Veronica will accept any apparel specific to the Brotherhood of Steel.
 * Veronica will 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 you the unarmed move Scribe Counter.


 * ED-E
 * Occasionally, ED-E seems to be able to carry Faction specific Armor. (Such as NCR, Powder Gangers and Caesar's Legion).

Behind the scenes
Several remaining resources point to the game having at one point allowed for the Courier 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

 * Sometimes Companions may go missing, see the the Talk page for possible ways to get them back.


 * When fast traveling while a companion tries to reach to the player (e.g. after mountain climbing) the companions will start running towards the player's previous location. To fix this issue speak to the companion, or save and reload right after fast-traveling.


 * 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 melee.
 * One way to solve for it is to prevent them from being poisoned in the first place. Make sure they are set to ranged weapons before fighting dangerous poisonous creatures.  (Veronica, despite her complaints about using ranged weapons, is very good at Energy Weapons.) If the fighting is really bad, just have them stay put and tank the creatures yourself.  (At least you can use Stimpaks and Antivenom without ill effect.
 * Another way around this includes having hardcore mode turned 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.
 * Temporary solution is sending your companion home or to the Lucky 38 then recruiting them again.


 * Clipping issues in mountainous regions may cause companions (or the player) to be stuck within a rock. The companion is unable to follow until the player enters another area or uses fast-travel.


 * If the player runs out of a companion's AI-area while the companion is still vainly trying to reach the player, the companion will go into "Wait" mode instead of zoning to the player.
 * Be careful when entering an area populated with enemies such as caves because if you enter and your companions are too far away when you enter, they might be placed in the middle of a group of enemies and possibly die before you can get to them.


 * The option to dismiss companions to the Lucky 38 will disappear even before Mr. House becomes hostile to the player.(Verified on Xbox 360 12/09/10)

Unconfirmed bugs / isolated incidents

 * If the player recruits Boone, Veronica, Raul, Rex, and Ed-E they will get the achievement "The Whole Gang's Here".


 * It's possible that you can lose the ability to hire or dismiss companions that you've already previously hired -- the dialogue option simply doesn't show up. An apparently-related bug is that during certain sequences where your companions are not allowed to follow you (most notably when entering the Lucky 38 for the first time), your companions are ordered to wait but will not resume following you when ordered.

Fixed bugs (Patch 1.2)

 * Everything in ED-E's inventory will be lost when you leave it with the Brotherhood of Steel or Followers of the Apocalypse for upgrades. This is because 'old' ED-E is replaced by 'upgraded' ED-E.


 * If you have several companions in the Lucky 38 at one time and leave Weapons laying around they may start to fight each other and they will die due to the fact that they are no longer listed as an essential anymore.No solution as of now (Sandbox-AI caused human companions to attack non-humans if the later carried consumable goods)


 * 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 v1.2 companions no longer walk to their residence but are teleported immediately)