Eliza Leagan functions as a primary antagonist throughout the Candy Candy narrative, featured significantly in Haru no Yobigoe via flashbacks detailing Candy's life. Introduced as the daughter of the affluent Leagan family, her father appears comparatively kinder than her mother. Eliza partners with her brother, Neil Leagan, in antagonistic acts, primarily targeting Candice "Candy" White Ardley after Candy's adoption as Eliza's designated playmate at the Leagan home. Despite this intended companionship, Eliza subjects Candy to consistent cruelty.

Eliza possesses brown eyes. Her hair is depicted as brown in original illustrations but pinkish-brown in the anime adaptation, typically styled in curled arrangements adorned with ribbons, complementing her affluent status reflected in her preferred dresses.

Her personality exhibits pronounced arrogance, narcissism, and sadistic tendencies. She maintains a mean-spirited, hostile demeanor, particularly toward Candy, whom she deems socially inferior due to her orphan background. Eliza frequently employs lies, gossip, and rumor-spreading to undermine Candy, demonstrating manipulative behaviors and a volatile temper. She operates with entitlement and perceived privilege from her family's wealth, adhering to a "screw the rules, I have money" mentality. While generally lacking empathy, she shows genuine concern for Neil when he faces physical harm and forms strong attachments, such as to Anthony Brown. After Anthony's death, she blames Candy and attempts to uproot his rose garden to claim the roses as mementos.

Her antagonism begins during Candy's time with the Legans. Eliza and Neil orchestrate pranks and schemes against Candy, including locking her in a dark room during a ball to prevent her interaction with Anthony, falsely accusing her of causing Annie Brighton's riding accident, and framing her for horse theft – leading to Candy's near-exile to Mexico. They manipulate events to poison horses and implicate Candy; when Neil sustains a black eye from an intervening figure during this, Eliza immediately blames Candy.

At St. Paul's Academy in London, attended by Candy, Eliza, Neil, and the Cornwell brothers, Eliza persists in harassment. She publicly humiliates Candy by labeling her the "Barn Princess" and Anthony's "murderess," arranges a rigged violin recital intended to expose Candy's lack of skill, and disparages letters from Candy's orphanage. After Terry Granchester rescues her from a pitfall trap Neil intended for him, Eliza develops a romantic infatuation with Terry. This escalates her jealousy-driven schemes, including fabricating a stable meeting between Candy and Terry to get them caught by school authorities. Terry eventually confronts her publicly, spitting in her face, leading to her week-long confinement as punishment for protesting Candy's exoneration.

Later, visiting their great-aunt Elroy in Chicago, Eliza continues undermining Candy, falsely claiming credit for nursing Elroy and convincing her mother to expel Candy and Annie from the Ardley mansion. During a garden party she hosts, Eliza facilitates Neil's trap that locks Candy in a tower, forcing a dangerous escape. When Candy works as a nurse in Chicago, Eliza bribes a colleague named Nathalie to prevent Candy from attending a play. After Terry is hospitalized following a train explosion, Eliza monopolizes his care and manipulates hospital assignments to remove Candy from the surgical ward. Discovering Candy sheltering the amnesiac Albert, Eliza contacts police to have Albert arrested, jeopardizing Candy's nursing career. In their final depicted interactions, Eliza announces Broadway ambitions to visit Terry, persistently attempting to provoke Candy, while Neil shows signs of reluctance to continue their antagonism.