Annie and Hallie are very similar in personality. They are both mischievous and intelligent, devising many schemes to torture Nick's fiancée, Meredith Blake, during their camping trip. They also share similar interests, such as their love for poker, fencing, and Oreos with peanut butter. They are both equally good at fencing and poker but Annie is slightly better at fencing than Hallie, and vice versa. They also are both allergic to strawberries.
However, Hallie still has distinct differences from Annie. She is less mature, as shown during their first meeting where Annie had bested her in fencing and she accidentally fell into a water holder. She was vindicated and immediately pulled Annie down in retaliation (despite Annie attempting to apologize and help her up), instigating the animosity between them. She also stole Annie's clothes after the latter, who lost in a game of poker, was forced to skinny dip in the nearby lake, prompting the prank wars that began between them. Hallie is slightly childish, as she still carries a stuffed animal named Cuppy who has been with her since she was young. She also has a bad habit of biting her nails where Nick says she has "been biting them since she could chew". Hallie is also impatient and does not listen, as she quickly hung up on Annie when she was trying to explain that Nick was remarrying, with Hallie brushing her off by saying it would not happen and that she wanted to spend more time with Elizabeth. She speaks in a less formal manner than Annie and shows an interest in wine, which stemmed from living in a vineyard with Nick. Hallie also has a much bigger appetite than Annie where the latter reprimands her to "stop thinking about your stomach at a time like this".
Hallie is a red-haired, blue-eyed Caucasian female of average height with freckles and pale white skin. She has short hair that reaches her shoulders and side-swept bangs that reaches below her cheek. She usually ties her hair into her signature half ponytail, pulls it all into a bun, wears a headband, or clips her bangs away. She also has pierced ears and painted nails, traits that originally set her apart from Annie before they hatched their plan to switch places, which forced Annie to have her hair cut short and ears pierced as well. Like Annie, she also wears a locket with her initial ("H") on it, which she has had since she was born. She dresses in more casual, colorful, tomboyish clothes in comparison to Annie, often wearing coats, t-shirts, and pants. She speaks with a California accent but is able to imitate a British one.
Annie and Hallie were also portrayed by Lindsay Lohan's double, Erin Mackey.