Elda Marker is the grandmother of the series' protagonist, Karin Maaka, and the mother of Henry Marker. Physically, she is a petite woman of short stature with a slight frame, long limbs, and soft features. She has large eyes and long, well-kept hair. In the anime adaptation, her hair is pink, and she is frequently depicted without shoes. Despite her small and demure appearance, she carries herself with a profound awareness of her own power. Her overall look is one of dignity and care, often contrasting with her more acrobatic and unpredictable movements.

Elda is defined by a dualistic nature; she is a powerful and often violent traditionalist who is fundamentally motivated by love and a deep-seated fear of loneliness. Her personality is a product of a traumatic history, having survived a near-extinction event that wiped out most of European vampiredom, including her original family. This past has left her with a fierce hatred for humanity and a strong distrust of anyone outside her immediate bloodline. She is depicted as being extremely self-centered, impulsive, and ruthless, often suggesting lethal solutions to problems without hesitation. This violent and conservative nature has earned her a fearsome reputation among other vampires, who cower at the mere mention of her name. Despite her ferocity, she is also prone to periods of deep grief and chooses to sleep for years at a time in her coffin, partly out of boredom with living and partly to be with her husband.

Her motivations are centered on the protection and preservation of her family, the Markers. Her profound love for her husband, James Marker, is a cornerstone of her character. She is deeply protective of her son, Henry, to the point of threatening violence against anyone who harms him. Her relationship with her daughter-in-law, Calera Maaka, is marked by a stereotypical and intense conflict; she has stated she disapproved of the marriage and has even tried to kill Calera on multiple occasions. In contrast, she is affectionate and doting toward her grandchildren, particularly Karin, for whom she worries greatly. She has a history of smothering her eldest grandson, Ren, with her rough brand of affection during his childhood, which left him traumatized for life.

Elda plays a pivotal role as a disruptive force and a source of both conflict and power within the story. Her awakening from a long sleep throws the Marker household into chaos. She initially runs rampant, hunting humans for blood to revive her husband, only to discover he has dried out and died, leading her to return to her slumber in grief. When she awakens again, she confronts the half-vampire Yuriya Tachibana, revealing her intense prejudice against half-breeds, whom she blames for betraying vampires to the Church in Europe. Her development is subtle but significant; despite her hatred for humans, she refrains from killing Karin's human love interest, Kenta Usui, leaving the choice to Karin herself and remarking that she must be going soft. Her perspective is further challenged by the revelation of Karin's unique condition of producing blood instead of consuming it. In the climax, she demonstrates her ultimate loyalty and power by single-handedly defeating the antagonists who kidnapped Karin.

Elda possesses formidable abilities befitting her status as a vampire who has lived for over two centuries. She is considered the most powerful vampire in existence, capable of easily overwhelming entire families of vampires by herself. She demonstrates mastery over her own willpower to exert control over other vampires' familiars, such as their bats, and has been described as being able to deal with twenty vampires without difficulty. Her physical capabilities are immense, allowing for acrobatic and lightning-fast movements that are stealthy despite her power. Her primary weaknesses are her emotional attachments and her periodic need to enter long periods of sleep in her coffin. Much of this information is based on the source material, as the anime adaptation provides a different backstory and presents her as significantly less powerful than her manga counterpart.