"I make apocolypes jokes like there's no tomorrow. Too soon."
Ellie was born several years after the spread of fungal pandemic. As a result, she grew up in an oppressive military Quarantine Zone in Boston, Massachusetts, with little knowledge of the world before infection. Before she died, Ellie's mother, Anna, asked Marlene to watch Ellie. However, Ellie never really met Marlene until she was 13 years old.
According to Ellie, at one point in her life, she drove a car.
Having been raised in an environment where modern standards and values have deteriorated, Ellie is considerably rash, foul-mouthed, impulsive, and temperamental, and isn't fazed by the notion of using violence as a means to an end or profanity as a way of expressing how she feels. However, she manages to maintain a particular innocence as she has yet to see the darkest sides of human nature and retains a palpable trust in people. This serves as the juxtaposition to Joel's wary, morose, and cynical outlook on post-pandemic life. She apologized to the soldier scanning her moments before stabbing him in the leg and hoped to incapacitate him and Ramirez with non-lethal means.
Ellie is also perceptibly clever and witty, and will do whatever it takes to keep Joel and herself alive. She has good survival instinct and she can be resourceful. She takes orders from Joel, but all the while makes it clear she does not "need any babysitting at all".
Ellie is enthusiastic about the outside world, given her confinement to her quarantine zone during her childhood. She is obsessed with things she collects from others, illustrated through her interest in music, movies, books, and video games. She frequently remarks her amusement upon finding interesting collectibles throughout her trip, which often clashes with Joel's physical indifference.
Ellie seems to adhere to no religion, as seen when she claims that she goes "back and forth" when asked by Sam if she believes in an afterlife. She says that she would like to believe it, but admits "I … guess not."
Ellie also suffers from a case of monophobia (fear of being alone) and states to Sam that she fears losing others she cares about such as Joel. The cause of this case is possibly related to her estranged parents and the deaths of her best friend, Riley, and hired guardian, Tess. Coincidentally, both were victims of the fungus and both had gotten infected while in her company. Soon after admitting her fear to Sam, he too, joins the list.
Ellie showed difficulties in positioning herself as a young kid in an adults' eyes, and sometimes even tends to "supervise" adults. She also takes "justice" into her own hands on her relationship with anyone, and does not think she requires any adults' consent. This self-parenting trait is related to her time spent in an orphanage, where she had learned to fend for herself. An example is when Joel falls on an iron rod. Though not shown, she takes Joel (by horse) to an abandoned mall in order to find supplies to stitch his wound up. Afterward, she takes him to an abandoned house on a sled attached to her horse and continues to nurse him back to health, while at the same time getting them food, and medicine from David.
Initially, Ellie is fairly unskilled in techniques to survive. She cannot swim, a hindrance to herself since swimming is crucial, given the amount of water bodies in the locations she travels. She carries a switchblade, which is her only (and primary) weapon until she is trusted with a gun by Joel. Due to his doubt of her ability, Joel doesn't hand a firearm to Ellie until she saves him by shooting a hunter, and only afterward does he teach her how to use a hunting rifle. Later on, she becomes more proficient in fighting, sporting a number of weapons such as a pistol, rifle, bow, and explosives, along with her trademark switchblade.
Ellie is, in some occasions, a playable character, much in the same way Joel is throughout the game. She is at first armed with a bow, which she uses quite well. Compared to Joel, however, she has limited hand-to-hand combat skills, as she is able to, but cannot easily take down a full-grown man. Her stealth capabilities are similar to Joel's, and she even has a less-honed version of the Listening Mode that Joel has. She can take down a full grown man by stunning the person with a brick or bottle; she is quickly killed if she faces opponents head-on. She is relatively inexperienced compared to Joel, as she is often surprised in the storyline and is snuck up upon. As an NPC, she is still effective as a partner, since she can spot enemies for Joel (she will not be spotted unless Joel is spotted) and at times fight alongside him. On rare occasions she may give Joel ammunition or a Medical Kit.
Ellie's most important, and unique, ability is her complete immunity to the fungus. Due to a strange mutation in her brain that developed following her initial infection, she became immune to the bites and spores of the Infected. It is presumed that if she was properly studied, some form of cure or countermeasure for the infection could be created. This is never clarified, however, since the study would result in her death, which Joel would not allow.
Ellie is also able to play the acoustic guitar and sing, which she presumably learned from Joel.