Hallie is a red haired female of average height and blue eyes. She has short hair that reaches her shoulders and side-swept bangs that reaches below her cheek. Hallie usually ties her hair into her trademark half ponytail, pull it all into a bun, wears a head band 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. 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 accent.
Annie and Hallie are very similar in personality. They are both mischievous and intelligent, devising many schemes to torture their father'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 Hallie is slightly better in poker than Annie.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 box of water. She was vindicate and immediately pulled Annie down in retaliation (despite Annie attempting to apologize and help her up), instigating the animosity between the two. 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 her father says she's "been biting them since she could chew". Hallie is also impatient and doesn't listen, as she quickly hung up on Annie when she was trying to explain that their father was remarrying, with Hallie brushing her off by saying it wouldn't happen and that she wanted to spend more time with her mother. She speaks in a less formal manner than Annie and shows an interest in wine, which stemmed from living in a vineyard with her father. 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".
- Annie and Hallie were also portrayed by Lindsay Lohan's double, Erin Mackey.
- Hallie is allergic to strawberries like her sister Annie.