Casca is a central character whose life is defined by hardship, loyalty, tragedy, and resilience. Born as the youngest of six children in a poor peasant family, her childhood was marked by deprivation and the constant threat of war on the borderlands of Midland. At the age of twelve, she was sold by her family to a passing noble to ease their financial burden. During the journey to his manor, the noble attempted to assault her, and it was at this moment that Griffith, the charismatic leader of the Band of the Hawk, appeared. He offered her a sword and a choice: to remain a victim or to fight for her freedom. In a decisive act, she killed her would-be attacker and chose to follow Griffith, joining his band of mercenaries.

Within the Band of the Hawk, Casca's determination and skill allowed her to rise through the ranks to become a unit commander and Griffith's right hand, the only woman to hold such a leadership position. Her personality before the catastrophic events of the Eclipse is marked by her fierce determination, strong will, and deep-seated loyalty to Griffith, whom she idolizes as her savior. This devotion is so profound that it often manifests as jealousy and resentment toward anyone she perceives as a threat to her place beside him, most notably the swordsman Guts. Her initial reaction to Guts is one of open antagonism and distrust, as she resents the immediate affection and trust Griffith shows him, feelings he had never displayed toward her. Despite this, her strong moral standards are evident when she prevents other members of the band, such as Corkus, from murdering Guts in his sleep. Her personality is not one-dimensional; she is shown to be deeply embarrassed by her battle scars and her muscular, athletic build, feeling they make her unwomanly, yet she does not reject her femininity outright.

Casca's motivations are initially dominated by her need to be useful to Griffith and to live up to the purpose he gave her. In her mind, her entire existence is validated by serving his dream of obtaining his own kingdom. However, her role in the story deepens significantly through her complex relationships with Griffith and Guts, forming a complicated triad. While she harbors romantic feelings for Griffith that border on worship, they are largely unrequited, with Griffith seeing her as a devoted and capable soldier above all else. Over three years of fighting alongside Guts, her animosity toward him gradually transforms. After a pivotal battle where she is weakened by her menstrual cycle and falls into a river, Guts saves her, and they take shelter in a cave. In an act that mirrors her first order from Griffith to warm Guts with her body, Guts returns the favor, leading to a conversation that builds mutual respect and attraction. This bond solidifies when Guts single-handedly holds off one hundred mercenaries to allow her to escape, shifting her feelings from rivalry to deep affection.

Following Guts's departure from the band and Griffith's subsequent imprisonment and torture, Casca's role evolves dramatically. Forced into leadership, she takes command of the remaining Hawks, demonstrating superb strategic and leadership skills as she leads them to safety from a deadly ambush, even after being struck by multiple arrows. When Guts returns a year later, her motivations become torn between her lingering sense of duty to the broken Griffith and her newfound love for Guts. She finally consummates her relationship with Guts, becoming pregnant with his child, yet chooses to stay behind to care for the crippled Griffith, feeling she cannot abandon him. This decision leads directly to the Eclipse, where Griffith, transformed into the demonic member of the God Hand known as Femto, sacrifices the entire Band of the Hawk. In an act of ultimate cruelty aimed at breaking Guts, Femto sexually violates Casca before him. The trauma of this event completely shatters her psyche.

Post-Eclipse, Casca's development takes a tragic turn. The once-ferocious warrior regresses to a childlike, pre-verbal state, losing all memory of her past self and her ability to speak. She bears the Brand of Sacrifice on her left chest, a curse that marks her soul for demonic persecution. In this vulnerable state, she becomes the primary motivation for Guts, shifting his goal from mere vengeance to finding a way to restore her sanity. Despite her mental regression, flashes of her former abilities and instincts remain. When threatened, she can react with lethal force, as seen when she instinctively kills three bandits who attempt to assault her. She is cared for by new companions, particularly Farnese, who acts as a surrogate caretaker, and while her journey becomes one of survival and protection, the underlying narrative is a slow, painful quest for healing.

As a fighter, Casca was highly skilled, considered the third strongest in the Band of the Hawk, surpassed only by Guts and Griffith. Lacking the raw strength of her male counterparts, her fighting style relied heavily on agility, finesse, and tactical knowledge. She favored a lighter short sword and employed nimble acrobatics, flipping and somersaulting to find weak points in her enemies' armor, such as joints and arteries, to deliver precise, fatal strikes. Even after the Eclipse reduced her mental state, this physical prowess and muscle memory are not entirely lost, surfacing as momentary, instinct-driven bursts of deadly efficiency. Her journey from a sold peasant child to a respected commander, and then to a victim of profound trauma whose recovery becomes the focus of those around her, makes her an essential figure in the narrative.