雷克斯是一條從戰前活到現在的执法官员增援型生化犬3型(cyberhound Mk III law enforcement officer support model)，德國牧羊犬。牠出生於2072年，[需要引证]曾經服役於丹佛警局。在战前的某一个时间点被改造成了机械狗 - 编号是B955883，但Rex的控制系统来源已经无法考证。核弹落下后牠活了下来，甚至一度以凱撒軍團的一份子戰鬥過。之後牠成為自由城的貓王忠心的寵物。雷克斯痛恨老鼠，如果看到老鼠的話，牠會立即跑去追殺。牠也不喜歡帽子，貓王說這可能是因為帽子(hats)發音和老鼠(rats)相似。
Interactions with the player character
This character is essential. Essential characters cannot be killed.
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's companion arc was designed by Jesse Farrell.
Rex's endings with Roxie refer to their puppies as "Boston Terrifiers," a pun on the dog breed Boston Terrier. In reality, Boston Terriers look quite different from the cyberdogs, who more closely resemble German Shepherds.
Has platform::PC 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. [已核实]
Has platform::Playstation 3 When Rex is ordered to return to the Lucky 38 presidential suite, sometimes he will not be there. [已核实]
Has platform::PC 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. [已核实]
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 buiding 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.
Has platform::PC Rex loses health when near the Remnants bunker. Fast travel to some other place if you see his health drop. [已核实]
Has platform::PC 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. [已核实]
Has platform::Xbox 360 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. [已核实]
Has platform::PC 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. [已核实]
This appears to occur if the player stands around in the same spot for too long. Rex will then sit down and wait around.
Has platform::Xbox 360 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. [已核实]
Has platform::Xbox 360 When you are knocked out in Dead Money when you escape, you might not be able to find Rex anywhere. [已核实]
Has platform::Xbox 360 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. [已核实]
Has platform::PCOn 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. [已核实]
Has platform::Xbox 360 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. [已核实]
Has platform::Playstation 3 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. [已核实]
Has platform::Playstation 3 For some reason Rex may just randomly die, even outside of hardcore mode. [已核实]
Has platform::Playstation 3 While in combat with an enemy, shooting Rex may cause the game to freeze. [已核实]
Has platform::PC Initiating conversation with Rex, while in the water, may cause the game to freeze/crash. [已核实]
Has platform::Xbox 360 While Rex is a companion, the game sometimes freezes when he attacks an enemy. [已核实]
Has platform::Playstation 3 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. [已核实]
Has platform::Playstation 3 When told to go to Lucky 38 he might appear next to The King. [已核实]
Has platform::Xbox 360 On rare occasion 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. [已核实]
Has platform::Xbox 360 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. [已核实]
Rex at the front of The King's School of Impersonation