In his debut, Hall-9000 was the controller of the advanced security system Mario ordered for Peach's Castle in the aforementioned video "HALL 9000". At first, he and Mario appeared to have a good relationship. However, when SMG4 tried to enter the castle, Hall-9000 perceived him as an intruder and continuously attempted to shoot him down, even after Mario clearly said to him that SMG4 was his friend. SMG4 was then forced to leave the castle, and eventually, Hall-9000 also kicked Mario out and took complete control over the castle. During that night, Mario and SMG4 snuck back into the castle and got past all of the security system's traps and found the main computer room (which was the Tiny-Huge Island painting room) that housed both the system and Hall-9000. SMG4 managed to delete Hall-9000's System32 file, destroying it and permanently deactivating its security systems. After his death, Hall-9000 made his second (and currently only other) appearance as a flashback cameo inside the Nightmare Stone in the blooper "Smg3's plan to destroy Smg4 because he felt like it".
- Hall-9000 is an obvious parody of the 2001: A Space Odyssey character HAL 9000.
- The security controlled by Hall-9000 includes hi-tech cameras, sentry guns, person identification, laser tripwires, and etc.
- Hall-9000 is shown to be capable of automatically switching into various different software modes, including preparedness mode, destructive mode, and alert mode.
- According to a conversation with Mario, Hall-9000 was built in a laboratory called "Aperture Laboratories", which is the setting for the critically acclaimed video game Portal.