Big Sister serves as guardian to three children—Loo, Sap, and Pom—on the primitive planet Cronkee, fourth world in the Unicorn system. The children address her as "Momma," cementing her caretaker role within their prehistoric-like society. Her history before this guardianship remains unrecorded.

After Spartan-1337 crash-landed on Cronkee, Big Sister had no immediate interaction with him. Her involvement triggered when the genetically modified Covenant brute Pluton arrived to eliminate Spartans. Pluton initially overpowered Spartan-1337, leading the children to relay the encounter to Big Sister and her companion Guy. Big Sister and Guy then collaborated with the children to support Spartan-1337 against Pluton. During the clash, Big Sister deployed a weapon system from a wrecked UNSC ship, firing a Magnetic Accelerator Cannon (MAC) at Pluton. The blast hurled him into an accidental Slipspace rupture—previously formed by Cortana’s manipulation of the ship’s drive—neutralizing the Covenant threat.

Post-conflict, Big Sister heard the children recount their collective adventure. She maintained no further documented contact with Spartan-1337 upon his departure. Her protective bond with the children and adept use of available technology during the Pluton incident define her known actions. No other media chronicles her activities beyond this event.