Arla Fett was the elder sibling of Jango Fett, a renowned bounty hunter and Mandalore. As the sole daughter of their Journeyman Protector father, she held the distinction of being Boba Fett's long-missing aunt.
Arla's early life unfolded on the planet Concord Dawn, where she resided on the Fett homestead alongside her parents and her brother, Jango. The year 58 BBY marked a turning point, as the Mandalorian Civil War, a conflict between Jaster Mereel's True Mandalorians and Tor Vizsla's Death Watch, engulfed Concord Dawn. When the Death Watch pursued surviving True Mandalorians after a defeat, Arla's father bravely offered refuge to Mereel and his comrades on their farm. Tragically, the Death Watch located them, resulting in the death of Arla's father as her brother Jango was fleeing. As they targeted Jango, Arla witnessed her mother's courageous act of shooting and killing a Death Watch member with a blaster rifle, which ultimately led to her own demise. While Jango escaped and was eventually rescued by Mandalorians, Vizsla, determined to eliminate all witnesses, ordered the "removal" of Arla.
Consequently, Arla was presumed dead on Concord Dawn alongside her parents. However, unbeknownst to others, she was spared by the Death Watch and taken captive. They saw her as a valuable asset, branding their symbol onto her back. After a period of captivity, Arla embraced the Death Watch's cause, becoming an assassin. Her association with them ended when she was arrested for a shooting incident 10 years before the end of the Clone Wars. Deemed mentally unstable while in custody, Arla was confined to the Valorum Center, a mental institution on Coruscant, for an extended period.
In 19 BBY, three years following Jango's death during the Battle of Geonosis, Bardan Jusik, a former Jedi who had become a Mandalorian, visited the center in search of Separatist scientist Dr. Ovolot Qail Uthan. During his initial visit, Jusik passed Arla's cell, overhearing words reminiscent of the Mandalorian language. Later, during the Separatist invasion of Coruscant, Kal Skirata and Jusik returned to free Uthan. Bardan once more heard similar Mando'a-like words. Acting on instinct, the former Jedi decided to liberate the prisoner. When the group was subsequently picked up by Walon Vau in a LAAT/i gunship, Jusik revealed her identity as Arla Fett, Jango's long-lost and presumed deceased sister. Following the issuance of Order 66, Arla accompanied Jusik and Skirata's group to Mandalore, seeking refuge in an old storage room at their stronghold, Kyrimorut.
At Kyrimorut, attempts to adjust Arla's medication, initially prescribed at the Valorum Center, proved inadequate. Arla, without provocation, assaulted Jusik. After hearing Arla's story and her claims of forgetting everything, Jusik offered to use the Force to erase her traumatic memories in exchange for information about the Death Watch.
Following her capture by the Death Watch, Arla Fett initially harbored resentment towards her captors but eventually developed a sense of attachment. Instead of seeking escape, she embraced their cause as an assassin. Through Death Watch intelligence, she learned of her younger brother Jango's survival but chose not to reconnect with him. During her arrest and confinement at the Valorum Center, she was heavily sedated to prevent self-harm due to the guilt she felt for willingly participating in the group responsible for her family's demise. After Bardan Jusik rescued her and brought her to Mandalore, her medication was altered. While she became more lucid, her memories and guilt resurfaced, leading to an unprovoked attack on Jusik. Later, in a moment of clarity, Arla pleaded with Jusik to end her life to alleviate her suffering, but he refused. Instead, they reached a compromise: Jusik would erase her memories in exchange for Death Watch intel.
Arla spoke Concordian, the language of Concord Dawn, which incorporated loanwords from Mando'a. She admitted to being a "good shot" with a blaster, a skill that proved valuable in her career as a Death Watch assassin.
In the unreleased Imperial Commando 2, Bardan Jusik was intended to wipe Arla Fett's memory, allowing her to cope with her trauma. Consequently, she would have forgotten her identity as Arla Fett and the events surrounding her family, but she would no longer experience the pain of those memories. Arla was then supposed to marry Bardan Jusik. In Legacy of the Force: Revelation, Jusik mentions his wife, but it was never officially confirmed that this wife was Arla.
Arla Fett made her debut in the comic Jango Fett: Open Seasons, appearing as an unnamed sister in a flashback. Karen Traviss expanded upon her backstory, including her name, in the novels Order 66: A Republic Commando Novel and Imperial Commando: 501st.
The character of Arla Fett exhibited an in-universe manifestation of Stockholm syndrome, a psychological phenomenon where a captive develops positive feelings towards their captors.