- “Take the shot, O'Brien, now! You're the only one who can do it!”— Cal ordering O'Brien to kill the Prophet.
Cal-141 was abducted and conscripted into the SPARTAN-II program by Dr. Catherine Halsey in 2517 when she was just six years old. Cal was then taken to the planet Reach, and trained by the AI Déjà and Chief Petty Officer Mendez, along with the rest of the Spartan children.
At some point between 2542 and 2551 Cal-141 was given command of a small unit consisting of four Orbital Drop Shock Troopers for a mission of extreme importance. The mission was to assassinate a minor Prophet in command of all Covenant military operations in the system, and gather intel on ruins of unknown origin on the Covenant-held planet, Heian. The unit consisted of Dutch, O'Brien, Cortez and Checkman. The unit planned to infiltrate the planet by exoatmospheric insertion using SOEIVs, masked alongside a common meteor shower. However, Checkman was killed in the upper atmosphere during the insertion. Once on the ground, the plan was to hike to an elevated position overlooking a landing zone for the Prophet, and to eliminate the target from long range using a sniper rifle system, giving the squad enough time to escape.
During the operation, a Jiralhanae Chieftain wielding a Gravity Hammer unexpectedly engaged O'Brien. Cal caught the Chieftains hammer by the handle, stopping its swing, a feat of unprecedented strength even by SPARTAN-II standards. Cal then engaged the chieftain in close-quarters hand-to-hand combat knocking it out of the forest with a kick and into a shallow stream overlooking a steep waterfall. She closed the distance while simultaneously dodging multiple swings from the Chieftain. Cal managed to deliver a powerful punch to the Brute's bare stomach, causing it to vomit out half-digested material. She then proceeded to sidestep the next hammer-swing and elbowed the brute across the face shattering a piece of its helmet and crushing one of its eyes; followed by another kick to the face causing blood to spew out from its mouth. As the Chieftain was momentarily stunned from the damage, she gave one final kick sending the Chieftain tumbling over the waterfall to its probable demise. Meanwhile, O'Brien had been flung over the edge to hang for dear life over the waterfall as they fought, and was rescued by Cal just before he lost his grip, but not before losing his own sniper rifle to the roaring rapids.
Cal and her unit had made it to the sniping position, and were waiting for the minor Prophet to arrive at the designated landing zone. The Prophet's transport descended, and a contingent of Sangheili Honor Guardsmen stood on high alert. Cal-141 set down her sniper rifle, drawing and connecting a cable to feed optical data from the scope to her helmet. At this time, the heavily-wounded Jiralhanae Chieftain that Cal had defeated the day before had survived, and moved to ambush Cal-141 and the three ODSTs. Cal spotted the Chieftain, about to strike O'Brien with a lethal swing, and quickly moved before the others could react to get the ODST sniper out of harm's way. The Chieftain viciously struck her across the head and right shoulder with its Gravity Hammer, nearly shattering her helmet's visor and resulting in fatal injuries, an attack too powerful for her armor system to absorb.
Dutch used this opportunity to rush the Chieftain, toppling it over in surprise. He lifted his M7 submachine gun and repeatedly shot it in the face whilst crying out in anger. O'Brien quickly ran to remove Cal's helmet to inspect her injuries. They were stunned to find that Cal was female, as opposed to the squad believing she was a male, likely due to her bulky armor and remarkable strength. Cal, realizing that with her badly damaged arm, as well as her blurring vision due to her head injuries, she would no longer be able to take the shot, and ordered O'Brien to kill the Prophet. O'Brien is able to quickly take the rifle, and expertly shoot the Prophet through the head with a single shot, to the dismay of the roaring Sangheili Honor Guardsmen at the site.
After O'Brien assassinated the Prophet, the ODSTs tried to tend to Cal's wounds, but she knew that she was too gravely wounded to make it back to the Destroyer. In her dying words, Cal asked Cortez to make a delivery to Dr. Halsey. This delivery consisted of a data crystal chip from Cal's helmet, which contained intelligence of the planet the mission was carried out on, as well as ruins that were neither human nor Covenant in origin (possibly Forerunner or ancient space-faring human). Cortez tried to assure her that she would be able to give the chip to Halsey herself, but Cal replied that the two of them knew it was not going to happen as she felt her death already moments away. She held Cortez's hands a bit longer and eventually died before the three ODSTs.
Cal's death left a profound impact on the ODSTs with whom she fought with on the mission. They, like all ODSTs, had a bitter rivalry with the SPARTANs and considered them to be freaks, but Cal's efforts in the mission and her actions in saving O'Brien's life many times until the loss of her own had changed their thoughts and opinions on the supersoldiers. This change was shown well as the ODST team later returned to their ship, where both Cortez and Dutch angrily scolded a marine for mocking Cal's combat credentials, and O'Brien pondered on how he might be dead if it hadn't been for her.
Cal-141 was likely classified as MIA, as per ONI policy regarding the death of SPARTANs, maintaining the belief that SPARTANs were invincible, to improve morale among the UNSC and its rapid loss of ground to the Covenant Empire. It is possible that in the end, O'Brien, Dutch, and Cortez buried Cal rather than taking her to the extraction point, as her body is not seen in the same place where she died, and they did not possess heavy-lifting gear required to move her, as SPARTANs weigh approximately 450kg (or 1000 lbs) in full MJOLNIR powered assault armor. It is also possible that they could have informed their superiors to her fate, and she could have been recovered later, but as the planet was Covenant-controlled this scenario is unlikely. It is unknown if the Mark IV worn by Cal-141 contained a fail-safe detonation feature as seen in the Mark V; if so, it is possible that the ODSTs could have been instructed to do so to deny the Covenant access to her armor by destroying it.
- She is one of the main characters of The Babysitter, a story focused on a UNSC mission to assassinate a Covenant leader, and the feud between the SPARTAN-II Project and the Orbital Drop Shock Troopers. It is one of Seven short films of the anime adaptation Halo Legends.