Abby is the female lead of the story in the American film version Let Me In; the character whom she was partially based on is Eli, one of the main characters in the original novel and the subsequent Swedish film of the same name. Both characters are vampires in secret and while they share many similarities, Abby was created to be her own character just as the American film was inspired by the novel rather than being an actual adaptation, according to the American film's director Matt Reeves.
In both the original novel and film adaptation, Eli is portrayed as a vampire who hails from Sweden, having survived for more than 200 years as a castrated boy who disguises himself as a girl. Abby, the female lead In the American version of the story, is based on her Swedish counterpart to a certain degree, although there is significant variation between the two. Unlike Eli, Abby was born and raised on the North American continent, presumably before the formation of the United States of America. Chloë Grace Moretz, the actress who portrayed Abby, states that her character, permanently frozen at the physical age of twelve due to the effects of vampirism, was born two to three centuries before the events in the film, placing the year of her birth around 1671 at the earliest. Furthermore, Moretz also revealed that her character was the daughter of a plantation owner, thus confirming the character of Abby to be genuinely female as opposed to the castrated Eli who apparently adopted the appearance of a young girl as a disguise.
Further variation between the two characters can be seen through Håkan, Eli's companion, and Thomas, the servant of Abby. While both pose as the presumably single father of a young girl, their reasons for doing so are quite different. Håkan serves Eli due to his desires as a pedophile; Thomas's loyalty to Abby is implied to be the result of genuine feelings for her rather than lust, as shown in the American film and its comic book prequel. Also, Håkan was an adult by the time he came to serve Eli, whereas Thomas had known Abby ever since he was a child.
In the novel, Eli is a roughly 200 year old vampire. Born sometime around the 1760s in Norrköping, Eli was originally a young peasant boy named Elias, who was taken from his family by "The Man in the Wig", a vampire nobleman who castrated and imprisoned him. Eli escaped, and adopted a feminine appearance as a result and is perceived by outsiders as a young girl. By the twentieth century, Eli manipulates an older man named Håkan, a former teacher and pedophile, into becoming a servant responsible for procuring blood from the living. By the end of book, Håkan dies and his role is passed over to Oskar, a young boy who became infatuated with Eli.
Lindqvist's novella Let the Old Dreams Die reveals that Oskar and Eli make a blood pact shortly after leaving Blackeberg, thus infecting Oskar with Eli's condition. By 2008, the two have traveled to Barcelona, Spain, and are pursued by a police officer who has become obsessed with solving the mystery of Oskar's disappearance.
In the Swedish film adaptation of the novel, the character of Eli remains largely similar to his portrayal in the book, although the details regarding his background are intentionally vague, leaving the character open to personal interpretation. Oskar first meets Eli meet in the playground where he spends most of his time. Despite Eli's desire for solitude, the vampire reluctantly befriends the young boy. They soon find common ground in their mutual fascination with puzzles, particularly a Rubick's Cube. Following the death of Håkan, Eli spends more time with Oskar and agrees to go steady with him. Eventually, Oskar discovers Eli's true nature as a vampire when he attempts to form a blood pact between them. Eli, having not fed for several days, is unable to resist licking the blood that falls on the floor. Though initially disturbed by this, after Oskar runs away from the house of his negligent father refuses to stay away, and the two become closer as a result.
When Lacke later tries to kill Eli in order to avenge the deaths of Jocke and Virginia, Oskar stops him at the last moment, before Eli kills and feeds on Lacke. After Lacke's death, Eli has to leave Blackeberg in order to avoid being caught by local residents and authorities, leaving Oskar back in a state of loneliness and misery. However, Eli later returns to rescue Oskar from being either maimed or drowned by a group of bullies at the bathhouse, killing three of them in the process. The two then gather their belongings and flee Blackeberg on a train together.
According to Chloë Grace Moretz, Abby was born two to three hundred years before the events in the American film, Let Me In, which is set in 1983. The actress states that her character was the young daughter of a plantation owner and that their family was hard-working but destitute. As such, they were forced to rely on Abby's wealthy uncle, a secret vampire for financial support. Moretz reveals that it was Abby's uncle who was responsible for infecting her with vampirism, as depicted in a deleted scene. The scene reveals how Abby was attacked and infected by her uncle when she was still a young child, thus freezing her at the physical age of twelve permanently.
Prior to the events of the film, Abby met and befriended a young boy named Thomas. At one point during Thomas' childhood, he and Abby posed together in a black and white photo strip which he kept for the rest of his life. He eventually became Abby's companion for many years and protected her while she slept during the day. At night, he stalked and killed numerous humans, particularly those who were vulnerable by being alone at night. After killing his victims, Thomas would then drain the corpses of enough blood in order to sate Abby's vampiric thirst.
The film includes an official prequel in the form of a graphic novel, titled Let Me In: Crossroads. Throughout the mini-series, Abby and Thomas hide in the rural countryside, living a secluded life while Thomas poses as Abby's reclusive father during the day; at night, he continues to provide Abby with the blood of numerous victims, usually random drifters, although he is also cautious of drawing too much unwanted attention to his actions. By this time, Abby gradually befriends her neighbor's son, a young boy named Jon Dixon. Thomas becomes increasingly jealous of their relationship and, convinced that Abby seeks to replace him with a younger companion, nearly beats Jon to death with a flashlight. With Jon covered in his own blood and on the verge of death, Abby is unable to resist her urge for fresh blood, and thus proceeds to feed on Jon, inadvertently infecting him with the vampire strain. Yet Jon's time as a vampire is short-lived. A salesman, who becomes a serial killer by this point, sets Jon on fire while trespassing in Abby's home. After Jon burns to death, an enraged Abby avenges her fallen friend by killing the salesman. With their hunting ground compromised, Abby and Thomas immediately gather their belongings and relocate to Los Alamos, an American city in the State of New Mexico where the film takes place.
Shortly after arriving in Los Alamos, Abby and Thomas, still posing as father and daughter, move into an apartment complex where a twelve year-old boy named Owen resides with his divorced and single mother. Owen takes an immediate interest in his new neighbor, but Abby is apparently hesitant to form any new relationship because of her true nature. However, she begins to bond with the bullied loner, despite of her reluctance to do so, thus causing them to spend more and more time together, much to Thomas's chagrin. At the same time, Abby is forced to take matters into her own hands after Thomas botches up an attempt to acquire enough blood to satisfy her needs. In order to survive, Abby attacks and kills one of her neighbors, an act which infuriates Thomas, but nevertheless hides the body in order to protect the female vampire. Knowing that his second failure could potentially compromise Abby's security, Thomas willingly burns his own face with acid to prevent the police from identifying him. When Abby visits him from outside a hospital window, she is visibly saddened by her companion's condition. In his final act of service as her familiar, Thomas offers his own blood to Abby, who accepts by gently biting into his neck. Due to the sudden loss of blood, Thomas lost consciousness and fell to his death.
The loss of Thomas causes Abby to focus her attention on Owen, who becomes increasingly infatuated with her. But when he seeks to solidify their relationship with a blood pact, Abby is nearly consumed with desire and only narrowly avoids attacking Owen by feeding on Virginia, a woman who lives in the same complex with her boyfriend. Although Owen is shocked, frightened, and generally unsure of what to make of the revelation concerning Abby, he comes to accept that he depends on her and the two become closer as a result. One morning, a detective's investigation into Thomas's killings leads him to Abby's apartment where Owen had spent the night. Upon entering the bathroom where Abby slept. Unaware of her weakness to sunlight, he begins to remove the black-out tape covering the windows, causing Abby's skin to crack and burn as the light shines on her leg. Her life is narrowly saved by Owen, who intervenes in order to save Abby's life. Taking advantage of the Owen's distraction, Abby's feral instincts cause her to pounce on the detective. Upon overwhelming him, she chews into his neck and proceeds to drain his blood, causing the detective to die in agony while a distraught Owen closes the door, unable to watch.
Realizing that her sanctuary in Los Alamos has been compromised, she quickly disposes the detective's remains before bidding Owen farewell as she leaves by nightfall in a taxi cab. However, she ultimately decides against leaving Owen and upon finding him at his school, she discovers his life to be in danger as a group of bullies attempt to drown him in a pool. Crashing through the window's ceiling, Abby violently slaughters the boys, literally ripping their bodies apart and tossing the pieces into the now blood-stained water. Reunited, Abby and Owen depart Los Alamos together.
In the final scene of the film, Owen is sitting in a train, guarding Abby's trunk. Due to the daylight, Abby hides within the trunk and taps a Morse code signal to Owen, to which he replies before humming his favorite song "Eat Some Now, Save Some For Later". According to the director and cast, the fate of Abby and Owen is intentionally ambiguous and left to viewer interpretation. However, the personal opinion of Kodi Smit-McPhee (the actor who portrayed Owen) is that Abby will eventually find a new familiar to do her bidding when Owen becomes too old to serve her, thus indicating that Owen (like Thomas before him) would ultimately be cast aside.
Although John Ajvide Lindqvist, the author of the original story, wrote the short story Let The Old Dreams Die as a sequel, it is important to note that it technically applies to the Swedish version Let The Right One In. According to Matt Reeves, Let Me In is largely inspired by Lindqvist's novel and therefore based on it to a certain extent, but still exists as its own story that is separate from the Swedish novel/film continuity. Hence, the sequel does not apply to the American film, leaving the ending to Let Me In vague as Reeves intended. Further evidence that distinguishes the American storyline from its Swedish counterpart is Let Me In: Crossroads. This graphic novel prequel, created specifically for the American film without any involvement or influence from Lindqvist, establishes a backstory for Abby and Thomas and details their experiences shortly before their arrival in Los Alamos. In short, "Crossroads" is the prequel to Let Me In, but does not apply to to Let The Right One In. By the same token, Let The Old Dreams Die is a sequel to the Swedish novel, but not to the American film, meaning that Eli and Oskar's fates are not necessarily the same as Abby and Owen's since they are all technically separate characters (i.e. Swedish and American versions respectively, with similar backgrounds).
In the film, Abby shows an interest in Shakespeare's Romeo and Juliet, quoting one of the lines ("I must be gone and live, or stay and die.") in a message she left in Owen's room. She also has a strong interest in puzzles, as seen by several different kinds of puzzle games in her apartment. This fascination also enabled her to solve a Rubik’s Cube in a single night, much to Owen's surprise and amazement. As a vampire, she possesses great physical strength, making her capable of ripping a human being into pieces when dangerously enraged. The comic reveals her other abilities, such accelerated healing, night vision, and the ability to fly without wings.
Abby's condition as a vampire includes several vulnerabilities as well, most notably the fatal weakness to sunlight. Slight exposure to the sun causes her skin to crack and boil; more exposure would result in her body becoming consumed with fire as seen when Virginia was exposed to daylight shortly after Abby infected her. As with sunlight, she is also vulnerable to fire and extreme heat. In addition to her high temperatures, another key weakness that Abby gained as a vampire is her inability to safely enter private residences, such as a person's home or room, unless she receives an invitation via verbal communication from the owner. For reasons unknown to herself, entering without permission causes her body to bleed with fatal consequences. However, the process can be stopped with the phrase "you can come in." Aside from this, she is free to enter public areas (i.e. stores, hospital lobbies, public schools, etc.) without the need for a verbal invitation.