Eli, looks like a young girl around the age of twelve, give or take. She is actually a 200 hundred and something year old vampire boy. In the novel she is presented androgynously and it explains her past. Centuries before meeting Oskar she was castrated by a vampire hunting nobleman. She is small and weirdly thin with dark shoulder length hair. When ever she doesn't eat she starts to look sick and her moods change. She's very intelligent and always soft-spoken. She doesn't enjoy being a vampire and she hates what she has to do to survive.

Upon meeting Oskar she lives with Håkan. Because Eli looks so young she cannot go out into society and fend for herself legitimately so she needs the help of someone older, someone like Håkan posing as her father. When Eli meets Oskar, he is sitting in the apartment complex play yard and she seems to be unbothered by the snow and cold. She comes off as being mean because she doesn't even want to get to know Oskar but as time goes on they start talking and Oskar offers her a Rubiks cube. She solves it and from there on out they become friends.

Eli has no friends and her only company is Håkan. When she meets Oskar she finds someone that she can finally be with. Eli understands that Oskar is somewhat weak and that he is often bullied at school. Eli believes that he should hit back, and hit back hard. She teaches him how to stand up for himself because she wants him to be strong. Eventually Oskar learns and he has her to thank. 

In the novel, Oskar begins to like Eli but she has a round about way of telling him that she's not really a girl... or a boy, either. She doesn't think that they can be together because she's a vampire. Not in the sense that she's afraid of killing him. No, that's never crossed her mind but she wants him to be happy. But Oskar just wants her and only her. He asks her to "go steady" with him but she doesn't say yes at first. She only agrees after learning that things won't really change between them.

Since she is a vampire there are certain rules that she has to follow...she isn't allowed out in the sun. If that were to happen her skin would break out in blisters and if she stayed long enough, she would eventually light on fire and be incinerated into ashes. To avoid daytime she sleeps in a bathtub layered in blankets with the windows completely blocked off. Another rule is that she has to be formally invited into someone's home. After she is let in, she can come and go when ever she pleases. If a vampire isn't invited it then they start to bleed from every orifice in their body. including their pores, eyes, ears, etc.

 The first time she goes over to Oskar's apartment she is faced with that problem. Oskar doesn't necessarily believe her and he asks her what would happen if she just simply walked in. So she does and then she starts to bleed. Oskar screams for her to stop and lets her in. Oskar then sees  that he can't do that. They begin to really love each other. 

Eli becomes the heroine in the story when she saves Oskar's life by taking the lives of the people that put him down. While he is at the pool he is approached by his bully's brother and threatened. She is always doing as much as she can to make sure Oskar is safe and she comes in and savagely kills everyone and saves him.

At the end, she is being kept in a box while traveling on a train with Oskar. They decided to go off and disappear together.