The character referred to as "The Father" (officially named Thomas in "Let Me In: Crossroads") in the American film adaptation Let Me In, and named Håkan in the original novel, and subsequent Swedish film of the same name, is Abby/Eli's human caretaker. Though it is everyone's natural assumption that he is her father, he is instead more of an employee, and she pays him to procure the life-giving fresh human blood that sustains her. While this sounds like a perfectly agreeable arrangement, the underlying story is far more sinister. In the American adaption, it is assumed that Thomas protects Abby because he had loved her since he was a child, and had still loved her until the end of his own life. In the Swedish novel, Håkan, as it turns out, is a pedophile, and his infatuation with Eli is quite perverse. Regardless of the fact that Eli is actually centuries old, all he sees in him is a child who needs his help, and the possibility of exploiting this need for his own sick compulsion. Despite this disgusting element of his personality, or possibly because of it, he is very protective of his "employer." When he was caught, he disfigured his face to make it difficult for authorities to trace him back to her, and ultimately fell to his death after he had allowed Eli/Abby to feed on him.
In the book, Håkan was not killed properly after being bitten by Eli before he fell, and so he returned as an undead shell of what he used to be, driven only by his lust for Eli and hunger for living flesh.
Håkan's pedophilia is not explicitly portrayed in the film, leaving his character in the film to interpretation.
As Håkan hit the ground in the film when he fell from the hospital window, it was shown that he still drew breath, hinting his reanimation, although he was never seen again after that point and no further references to the undead Håkan were made.
In the American adaptation, unlike how Håkan met Eli as an already grown man, Thomas ("The Father"), met Abby when he was a young adolescent, likely around the age of 12 or 13. While Håkan in the Swedish version agreed to procure blood for Eli on the condition that he would provide shelter for him and possibly satisfy his pedophilia, Thomas procured blood for her out of his personal loyalty to Abby.
In the comic book prequel, The Father is named Thomas.