Part German Shepherd, Rex is a "Cyber-Hound Mk III L.E.O. (Law Enforcement Officer) Support Model" that has been around since pre-War times. He was born as a regular dog in 2072[Non-canon 2] and served in the Denver Police Department. Sometime before the war, he was cybernetically enhanced and turned into a cyberdog - serial no. B955883. He survived the destruction of Denver and the harsh wastelands, eventually becoming Caesar's companion for a time, but was lost in battle.
He later reached Freeside, where he came under the ownership of the King, who loves him dearly. Rex does not like rats, or hats, according to the King. The King says his dislike of hats may come from the fact that hat rhymes with rat.
He is suffering from neural degeneration as a result of his aging brain. As the process will result in his death without intervention, the King has contacted the Followers of the Apocalypse, to ask for help but they do not have the capabilities to perform brain surgery nor the knowledge of cybernetics that is needed. However, the Followers do know that a Doctor Henry in Jacobstown has these expertise and is likely Rex's best hope.
If the player character has the Wild Wasteland trait, Rex will say "aroo" to the Courier at a random time. The Courier can then ask if Jimmy has fallen down a well (this is random and does not always happen). This adds the location Jimmy's well to the map. Down in the well, the Courier will be confronted by a mole rat, and there will be a skeleton. Next to him is the Abilene Kid LE BB gun, 2 tins of BBs, a super stimpak and a rawhide cowboy hat. This is a reference to the television series Lassie, as well as to the quest with Laddie in Fallout 2.
Having never received a replacement brain, Rex finally succumbed to old age, abruptly shutting down forever one quiet morning. After two centuries of life and decades of service to humanity, Rex collapses and dies.
Revitalized by Violetta's brain, Rex eventually learned to balance the memories of his old life with Violetta's experiences among the brutal Fiends. His mind had difficulty adjusting, but Rex eventually found peace with his new, more vicious self.
Complete Rex's personal quest, Nothin' But a Hound Dog, by implanting Violetta's Brain in Rex.
With the transplant of Lupa's brain, Rex gained all of the donor's experiences traveling with the Legion. These melded well with his own memories of the Legion, and his new mind quickly adjusted to the myriad memories.
Complete Rex's personal quest, Nothin' But a Hound Dog, by implanting Lupa's brain in Rex.
After Rey's brain was transplanted into Rex's cybernetic body, it took Rex some time to adjust to the old scrapyard dog's memories. Eventually, Rex's mind settled peacefully, melding his own memories with that of long travels with Old Lady Gibson.
There, she ran across Rex at the Kings School of Impersonation. His brain was failing. That was all right, he was still smarter than the Think Tank. The two of them constructed a litter of cyberpups, a small army of Boston terrifiers that gnawed and devoured anything in their path.
Roxie created and Rex is alive, but not obtained as a companion.
There, she ran across Rex, who had been following the Courier, just as she had. They barked for a while, and realized they had a lot in common. The two of them constructed a litter of cyberpups, a small army of Boston terrifiers that gnawed and devoured anything in their path.
Roxie created and Rex is alive and a companion on standby.
There, she ran across the corpse of Rex. She dragged him back to the Big Empty and had him rebuilt for company. The two of them constructed a litter of cyberpups, a small army of Boston terrifiers that gnawed and devoured anything in their path.
Because his "Bite" weapon engagement radius is set to that of a ranged weapon and Rex has a high Perception stat, he will often charge towards enemies from long distances, even if set to a passive behavior through the Companion Wheel.
If Rex dies, the King will have a few solemn words about his passing.
If the Courier has a hat equipped when first talking to Rex, he will start barking at them because of his hatred for hats. He will continue barking if the Courier or their other companion still wear a hat.
Like all cyberdogs, Rex is not affected by pulse weapons.
It appears that Rex will not attack giant rats or mole rats if the Courier has the Animal Friend perk. Likewise, despite his apparent hatred for rats, he won't attack Snuffles at Sloan or the occasional non-hostile giant rat running around Freeside.
Rex has a unique special attack where he actually tackles enemies to the ground.
Rex will growl uncontrollably as if enemies are nearby if you take him into the Silver Rush.
If you have him as a companion and go to recruit Arcade Gannon, Arcade will say: "A courier, a doctor, and a cybernetic dog. Maybe when this is over we can open an act at The Tops."
Even though Rex is a cyberdog, non-player characters still talk to him as if Rex is a normal canine or a real person.
He will bark at other people if he sees them wear a hat.
When Rex is a companion and the player character talks to him via the Companion Wheel, he will growl if they have a hat equipped. They can then talk to him about hats and convince him that they're "on the same side," and he will then no longer growl about their hat.
Rex will curl his tail and bark after the player character has manipulated his inventory.
Rex is Latin for "king," which relates to his owner's name.
Rex's endings with Roxie refer to their puppies as "Boston Terrifiers," a pun on the dog breed Boston Terrier.
In the course of the quest Nothin' But a Hound Dog, it is possible to stealthily kill Lupa and loot Lupa's brain from her corpse. If she is successfully killed without the player being noticed, Antony may not "notice" that Lupa has died and will still give you the option to fight her in the arena for her brain. If the player accepts, it is possible to obtain a second Lupa's Brain. Having two of the brains in your inventory will cause Dr. Henry to not recognize that you have the brain at all, making it impossible to give Rex Lupa's brain. To solve this, revert to an earlier save. Make sure that you only have one instance of Lupa's brain. [verified]
Sometimes simply entering the King's School of Impersonation will cause Rex to appear near The King and will start running to the Lucky 38. The player can chase and intercept Rex or wait until he returns to the presidential suite to recruit him. [verified]
If he doesn't appear near The King in his school, he may have already left the building as the player was entering, from here just exit the building and walk to the Lucky 38, DO NOT fast-travel there, you may encounter him as you're going there or in the Lucky 38 casino floor.
Rex loses health when near the Remnants bunker. Fast travel to some other place if you see his health drop. [verified]
If you dismiss Rex during Nothin' But a Hound Dog when he is theoretically on loan to you and not a permanent companion, he may disappear and not return to The King. He may eventually show up after traveling to Doc Henry and speaking to him about Rex. [verified]
During combat, Rex might chaotically run around from-and-to foes while not attacking anyone. He may also in fact not attack at all when you have him. [verified]
Rex will sometimes stop moving as if he has been told to wait. You will need to keep checking to see if he is still with you. This can usually be fixed by telling him to wait, then tell him follow you again, fast traveling or going through a door that requires a transition. However, these fixes are per-incident and he will randomly do it again. [verified]
This appears to occur if the player stands around in the same spot for too long. Rex will then sit down and wait around.
While doing the quest You Can Depend on Me (or just trying to sneak into the Gun Runners), Rex may glitch while in combat with the guards and end up in the Vendortron's kiosk. There is no physical way into the kiosk and Rex can still be spoken to. The only way to fix this problem is to fast travel to another location, in which Rex will appear beside you in patched versions of the game. [verified]
When you are knocked out in Dead Money when you escape, you might not be able to find Rex anywhere. [verified]
If Rex and another person (i.e. the player or another companion) attack the same enemy at the same time and the game goes into slow motion kill-cam, the game may freeze. Turning kill-cam off may solve this problem. [verified]
On PC, if Rex is running to attack an enemy and you or another companion kill this enemy while Rex is charging, the game will CTD with a visual c++ runtime error. [verified]
If you tell Rex to wait outside when visiting Caesar in the quest Render Unto Caesar and you confirm that you want to battle Benny, the game will automatically send you to the ring, and when you win, Rex will be nowhere to be found. He isn't with The King or at any casinos. If you were already at the point in Nothin' But a Hound Dog where you are looking for a new brain, you can still gain a brain as the AI will think you still have Rex, but when you go back to Doctor Henry, he will say he can't perform the surgery without Rex. To recover Rex on Xbox, you can go to Vault 22, fix the elevator (requires Repair 50) and use it to go to the 5th floor. [verified]
When acquiring Rex from The King in his room, very top floor of the King's School of Impersonation, he will sometimes drop through the floor and wind up at the bottom level. Just wait a few minutes and you will find him running towards you. [verified]
For some reason Rex may just randomly die, even outside of hardcore mode. [verified]
While in combat with an enemy, shooting Rex may cause the game to freeze. [verified]
Initiating conversation with Rex, while in the water, may cause the game to freeze/crash. [verified]
While Rex is a companion, the game sometimes freezes when he attacks an enemy. [verified]
While at the Lucky 38 with other companions, Rex will sometimes start to fight them. There is no known fix for this. He may also do this with Quartermaster Bardon at Hoover Dam. [verified]
When told to go to Lucky 38 he might appear next to The King. [verified]
On rare occasions Rex will begin to growl uncontrollably as though there are enemies nearby. He will continue to do this even after you load a new location. It seems that this sometimes happens when you load a previously saved game while Rex is in the middle of attacking an enemy. The only known way to solve this problem is to send Rex to the Lucky 38 and then recruit him again. [verified]
Sometimes, the people around you in a location will attack Rex until he is unconscious (casual mode). They will not be hostile to you, only Rex. Fast traveling will stop the attacks for a while, but not indefinitely. [verified]
↑Antony: "The mechanical dog you have with you - where did you get it?" The Courier: "Rex? I picked him up in New Vegas." Antony: "Oh? Such things are common in Denver, but I didn't know that other cities had them as well." The Courier: "Why do you want to know?" Antony: "I ask because Caesar once had a similar pet, but it was lost during a battle." (Antony's dialogue)
↑The Courier: "Absolutely. I'll see that he gets better." The King: "I can't thank you enough. Now, there's a few things you should know about ol' Rex here if you're going to be traveling together. First, he hates rats. Can't stand the things. Giant rats, molerats, doesn't matter. He catches a whiff of one and he's off like a shot after them. He's normally pretty obedient, but you might have to chase him some if he goes after one of the little varmints. Second, he doesn't like hats, or the people wearing them. Don't ask. I have no idea why. Maybe because it rhymes with rats. That should be it, though. Other than the occasional bad spot when his brain starts hurting him, that is, but I'm hoping you'll see to that. I wish you both luck. Take care of my Rex now, y'hear?" (The King's dialogue)
↑The Courier: "Something's wrong with my cyber-dog. I was told to come to you." Henry: "It's neural degradation. Bio med gel can only preserve a living brain for so long, so you'll need to find a replacement. I haven't left Jacobstown in years, but there was one woman in Novac... Gibson? I remember her living with a pack of hounds. Aside from her, I know that the Fiends and Caesar's Legion fight alongside dogs. There could be viable specimens among them, if you can get to them." (Henry's dialogue)
↑The Courier: "The King said you could tell me more about his robotic dog." Julie Farkas: "Rex? It breaks my heart every time I see him. He's such a good dog." The Courier: "So you can't heal him?" Julie Farkas: "No, Rex's condition is way beyond anything we can handle. He requires brain surgery, and some sophisticated cybernetics work, too." The Courier: "The King brought him here for treatment then?" Julie Farkas: "Yes, a few months ago. We had to tell him there was nothing we could do." The Courier: "So there's no chance he'll ever get better?" Julie Farkas: "I didn't say that. While no one here has that kind of expertise, I do know of one man who might fit the bill." The Courier: "Is there anyone who could do all that?" Julie Farkas: "There's an old scientist named Dr. Henry who reportedly specializes in this sort of procedure. He'd probably be your best bet. Last I heard, he was living up in Jacobstown, far to the northwest." (Julie Farkas' dialogue)