Casca was born the youngest of six children to peasants in a remote mountain village. Facing starvation and the threat of abduction, she developed a bleak worldview early on. At twelve, her parents sent her to work as a castle maid for a nobleman. During the journey, the noble attempted to rape her. Griffith intervened, slicing off the noble's ear and offering Casca his sword. She killed her attacker, an act that filled her with unease but led her to join Griffith's Band of the Falcon, where she learned to fight for self-protection.

Within the Band, Casca became the sole female soldier, rising to unit commander and earning respect as the third-strongest warrior after Griffith and Guts. Her troops affectionately called her "Big Sis" for her leadership and protective nature. She initially resented Guts, jealous of Griffith's immediate favor toward him. Over time, repeated life-saving encounters and forced cooperation fostered mutual respect and romantic feelings between Casca and Guts. Her loyalty to Griffith stemmed from his role as her savior, though she gradually shifted her devotion to Guts as their relationship deepened through mutual honesty rather than one-sided reverence.

Following Griffith's imprisonment by the King of Midland, Casca assumed command of the Band of the Falcon. She prevented the group's annihilation during their year as fugitives and led the mission to rescue Griffith from the Tower of Rebirth. After his rescue, Griffith's physical and mental deterioration culminated in activating the Crimson Beherit during the Eclipse. Griffith sacrificed the Band to become the God Hand member Femto. Casca survived the Eclipse but endured Femto's rape, inflicting severe psychological trauma. This caused her mind to regress to an infantile state, leaving her largely mute and unable to process complex emotions or memories.

In this regressed state, Casca displayed primitive empathy and motherly affection toward her demon child—a corrupted version of her unborn son with Guts, tainted by Femto's assault—and the Moonlight Boy, becoming distressed when they vanished or were endangered. She retained fragmented, fond memories of her time with the Band, though deeper traumatic memories were subconsciously suppressed. Guts later journeyed to Elfhelm to restore her mind. The Flower Storm Monarch oversaw the "Corridor of Dreams" ritual, where Schierke and Farnese explored Casca's psyche, repairing her fragmented mental state and restoring her prior personality and memories.

After restoration, Casca remained aware of events during her regressed years, recognizing companions like Farnese and recalling Guts with emotional complexity. However, she suffered crippling panic attacks and vivid flashbacks of the Eclipse whenever she saw Guts directly, a lingering effect of her trauma. This condition strained their relationship, manifesting as avoidance and distress in his presence.

Throughout her development, Casca balanced her warrior identity with her womanhood. She wore practical battle attire but secretly desired acceptance in both roles, a conflict highlighted by her embarrassment over her muscular physique during intimacy with Guts. Her resilience as a leader and fighter contrasted with vulnerabilities stemming from her traumatic past, including multiple attempted assaults. The Memorial Edition adaptation restored previously omitted scenes, such as the "Wounds" sequence, where she and Guts confronted his childhood sexual trauma during their first intimate encounter, reinforcing their emotional bond.