She is a nightkin who served in The Master's army, along with Lily Bowen. With the Master's army disbanded, she wandered aimlessly for years, at a certain point stumbling upon an old Mister Handy named Rhonda, with whom she fell in love with (probably due to long-term exposure to stealth boys, although it is assumed that her mental judgment was impaired to begin with).
At some point she joined Marcus, an intelligent first-generation super mutant, who founded the settlement on Black Mountain sometime in 2278. Using their lack of intelligence, Tabitha rallied all of the second-generation mutants against Marcus. This led to Marcus and the first-generation mutants to leave, eventually discovering a pre-War ski lodge.
Tabitha is now the tyrannical ruler of all the mountain's second-generation super mutant and nightkin inhabitants, and has demanded that all humans be shot on sight. She hosts a radio show with her "lover" Rhonda, but it would appear that she is merely talking to herself as Rhonda is deactivated. She has kept a ghoul engineer imprisoned just so he can fix Rhonda and other things the super mutants wanted fixed. Rhonda remains broken however, as Raul is mechanically profound yet not scientifically inclined enough to repair the robot. Unlike Marcus, her dedication to the Master never diminished and she often speaks nostalgically about the days when she served under him.
If you choose to repair Rhonda and let Tabitha walk out of Black Mountain alive, her ending states that "Tabitha and Rhonda went east, through Caesar's land. Occasionally tales of their exploits found their way back west, though few believed them. Eventually the stories concerning the duo were collected and published, and proved to be quite popular with children."
If you choose to kill Tabitha, her ending states that "Though some super mutants and nightkin continued to journey to the legendary Utobitha, they found little trace of its existence. Some eventually found their way to Jacobstown, but many wandered off into the wastes, confused and disheartened."
If you free Raul, don't repair Rhonda, and don't kill Tabitha, her ending states that she, enraged by Raul's escape, "took the remaining nightkin of Black Mountain with her and vanished without a trace. Travelers in the region soon learned to avoid the roads at night, as caravans were repeatedly attacked by unseen assailants."
If you don't free Raul, don't repair Rhonda, and don't kill Tabitha, her ending states that "Black Mountain Radio continued to broadcast its peculiar form of propaganda. Raul Tejada faced his execution each day, though pardoned in the end. Travelers venturing too near Black Mountain continue to be harassed by Tabitha's followers."
Tabitha has a storage room at the top of Black Mountain. Upon entering the storage room there is a training dummy with a note attached to it (indicated by an arrow on the training dummy). The note says:
To Whom It May Concern
We got some pretty good stuff from the last raid and unlike last time, I don't want any incidents. The only logical conclusion was to hide everything throughout this room. if you can read this, you have my permission to use any items you can find.
Stuff I hid:
1 missile launcher
3 bricks of C4
5 frag grenades
1 super stimpak
Your Benevolent Supreme Overlord, Tabitha
Item locations are described by wall associated by compass direction or shelf row, shelf unit and shelf tier (whereas the first row is closest to the door, and last unit is farthest to the back wall).
According to references in the GECK, Tabitha may have been known as "Bridget" at some point during development.
If Rhonda is repaired, even though Tabitha leaves, her radio program continues to play until the player enters her room. It is only then replaced by static.
Her terminal in the lower level of where she is found states that she hides a key under the stairs located out the back door, which can be used to unlock her door.
Tabitha obviously has quite an obsession with keeping Raul. If the player frees Raul and escapes Tabitha's ambush, Tabitha will endlessly pursue the Courier. If the quest Crazy, Crazy, Crazy is still active, Tabitha and her movements will be visible on the map.
Even before meeting Tabitha, the player can get hints that all is not as it seems on Black Mountain through Black Mountain Radio. At one point, Raul will slip up, and almost call the "Rhonda" personality Tabitha. At another point, Tabitha will say "I still miss you Rhonda, after all these years," which is odd, as she is supposedly talking to Rhonda. An angry Rhonda also sounds exactly like Tabitha while yelling that humans should "stay the hell away".
She references the "battle cattle" as Caesar's Legion and the "two-headed bear people" as the NCR, because of their respective flags.
If Rhonda is repaired, the robot and Tabitha will walk down the mountain, taking the short cut under the broken pylon and down the man-trapped path. They will pass through the first check point, guarded by a player hostile super mutant and will disappear into thin air at the point where the road turns towards Neil's shack.
Before the game's release, an article in Official Xbox Magazine suggested that Tabitha was a male pretending to be a woman out of insanity.