Alicia Crystella is a human Imperial Knight from the ducal House Crystella within the Kingdom of Lyferia. She possesses long waist-length red hair, red eyes, a tall and thin build with slender limbs, and wears small edged glasses to correct vision damaged by excessive childhood studying. Her standard attire is lightweight, practical armor designed for mobility while highlighting her figure, featuring exposed shoulders and a mantle.

Her background involves severe parental abuse. Duke and Duchess Crystella enforced a rigorous "proper" education that impaired her eyesight, then berated her for requiring glasses. They routinely invaded her privacy, searching and destroying possessions deemed "improper" without consent, dismissed her career achievements, and threatened to arrange a marriage if she did not select a "proper" spouse herself. This abuse extended into her adulthood. Forced into becoming an Imperial Knight, she endured humiliation as the token female knight, including a stripping incident where she was chained naked and whipped by the Chivalric Knight Order. The cumulative trauma from familial and institutional mistreatment led to a psychological break, fueling intense hatred toward all mortal races, including herself.

Alicia maintains a meticulously crafted public persona as a kind, polite, and morally upright knight with exceptional communication skills and situational awareness, earning high regard from the Lyferian king and a reputation as his favorite knight. This contrasts sharply with her true nature: a demon-worshipping sociopath who considers mortal races "unsightly and weak" while idolizing Demonic Beings for their destructive power and unrestrained violence. Both personas share extreme seriousness and uncompromising dedication to any task undertaken. Her obsession with thoroughness manifests as a fixation on eradicating corruption completely, often envisioning extreme outcomes like decapitating nobles or burning entire kingdoms.

Her primary weapon is a sword. Though initially assessed as a warrior around level 40, she possesses noble combat training and wields expensive family-provided magic stones containing sealed high-level spells. She exhibits rare compatibility with Magimatic Sol technology—specifically the <<Goldinus of the Gold>> unit—which consumes most users but bonds with her after her capture by the Gelmed Empire, resulting in her enslavement via a collar that forces her to serve as a pilot for their mechanized weapon.

Alicia's story arc involves orchestrating the revival of the Demon Lord Krebskulm to exterminate all mortal races. She manipulates events by supplying Galluk with a cursed sword to summon the demon Gregore in Faltra City and later betrays protagonist Diablo's group by delivering Rem Galleu and Krebskulm to the sadistic paladin Saddler. Her intent is to traumatize Krebskulm into assuming a destructive true form, stating she would willingly die if it achieved this goal. When this plan fails due to Diablo's intervention and Krebskulm's refusal to annihilate humanity, Alicia attempts suicide in despair. Diablo prevents this and redirects her hatred toward Lyferia's corrupt nobility and clergy, assigning her to gather intelligence for their overthrow. She subsequently pledges undying loyalty to Diablo, interpreting his actions as a path to systemic destruction and declaring herself his "possession" to be disposed of at his command.

Post-redemption, she aids Diablo's allies during the Gelmed Empire's attack on the Lyferian capital, defending a church bridge alongside Rem before her capture. She later secures the safety of Horn and Sylvie within the church sanctuary and establishes an information network to track Diablo's movements during his absence. Her relationship with the Fallen Edelgard evolves into a mutually protective bond, and she collaborates with Diablo to heal Edelgard after her injury.