At times, Brittany has been shown to be impulsive. However, she drives toward any goal she has set with complete determination, stopping just short of ruthlessness. Brittany is depicted as domineering and as somewhat sarcastic and untrustworthy. An example of this is when she dismisses Simon's advice in The Chipmunk Adventure, when he advises them to avoid the easterly route to avoid a hurricane. She believes it to be a trick to slow them down so they'll lose the race. Also, due to her ego, Brittany has been shown to be unable to admit her mistakes. Such as when her balloon is destroyed as a result of being caught in the hurricane. Her sisters demand that she admits her mistake, however as she is trying to admit it, she is unable to say the full sentence, instead changing the subject. However, Brittany is shown to be kind and compassionate, such as when she at first refuses to help Eleanor take home a kidnapped baby penguin, but later submits after seeing its deep sadness and longing.
Brittany has been shown to be vain about her appearance. In I Love the Chipmunks Valentine, it is shown that several boys wanted to go to the dance with her (although she only wanted Alvin) which means she is seen in boys' eyes as "attractive." Like Alvin, Brittany has always taken on the role as leader of the Chipettes, being lead singer or forward speaker for her sisters.
Alvin and Brittany have always shared an on / off relationship, often appearing to hate each other. However, they are known to express romantic feelings towards each other. The common scenario between the two is that at first they appear to be friends, but eventually butt heads, argue over the subject for a period of time (often blowing it out of proportion). Despite how bad things get, the two eventually agree that they care for each other and make up.
Brittany's age in regards to her sisters is never stated in the films. Despite not having a stated last name either, it is presumed to be Seville because of The Chipettes being taken in by Dave at the end of the second film. However, this naming would be controversial as it would make her Alvin's adoptive sister in addition to his love interest.
Though her basic concept is retained in the CGI films, there are some notable differences. Besides appearance, her personality is toned down. She is a lot friendlier and polite, making it easier for others to approach her. She is nicer and has better control over her temper than Alvin does, though it is still a bit shorter than most. She is also shown to have a much better, loving relationship with her sisters, becoming visibly concerned and defensive about them when Ian tried to split them up. Unlike her cartoon version, she is able to apologize when she has judged incorrectly.
Brittany's age in regards to her sisters hasn't been stated in the series so far, though she retains the last name Miller as stated in Jeanette's Secret Garden.
Brittany is less toned down from her 80s personality than she is for the CGI films. While it is clear she cares about others' feelings (such as when she decided not to tell Dave Alvin was planning to skip out on family day in Family Spirit), she does step on her sister's, primarily Jeanette's, emotions as seen in My Sister The Weirdo. Even so, she recognizes her mistakes and seeks to correct them in most instances (in Sister Act, their dispute is never settled). Most of her conflicts are still with Alvin.
She also exhibits a greater sense of responsibility by serving duty as Principal Meadows' assistant, a position that she takes very seriously - so seriously, in fact, that in The Temp, she sneaks into the school after dark to clean up Jeanette's mess when Jeanette fills in for her while she is recovering from a sprained ankle. Her idol is famous fashion designer and entrepreneur Marina Rodenchia.
In the Alvin and the Chipmunks series, she has her hair in a ponytail and blue eyes, and her signature color is pink. She is usually seen wearing a pink sleeveless short shirt over a darker pink sleeveless shirt, a pink skirt, a yellow scarf tied in a bow around her neck, periwinkle tights, light pink flats, and yellow legwarmers. In Alvin and the Chipmunks Meet the Wolfman, she no longer wears the legwarmers nor scarf to give her a more 'hip' look and wears different footwear.
In the CGI films, Brittany has blue eyes. In Alvin and the Chipmunks: The Squeakquel, she wears a light pink long-sleeved shirt with a pink leather jacket, black neck tie, and magenta skirt. In Alvin and the Chipmunks: Chipwrecked, she wears a blue denim jacket with a light pink long-sleeved (albeit with holes) shirt and pink skirt with black dots. In Alvin and the Chipmunks: The Road Chip, she wears a blue jacket with a red-and white-striped long-sleeved shirt and a red-and magenta-striped skirt with a matching belt.
In ALVINNN!!! and The Chipmunks, her hair is more yellow than previous incarnations held in a ponytail with a magenta ribbon. She has blue eyes with pink eye shadow as well as magenta lips and mauve nails. Her usual attire consists of a fuchsia-pink long-sleeved button-up cardigan with a white cursive 'B' over a lighter pink skirt, black leggings, and shiny magenta shoes with little bows. Brittany is revealed to have a heart-shaped scar on the sole of her left foot.
Jeanette isn't brave enough to stand up to Brittany. Brittany pushes Jeanette around a lot and takes advantage of her kindness but deep down they love each other very much and are very loyal to each other. Brittany gets very defensive of Jeanette when someone else is mean to her, in most situations.
Eleanor always stands up to Brittany when she tries to push Jeanette around and is generally the only person that tries to stand up to Brittany (besides Alvin). The two of them do share a very nice sisterly relationship.
Brittany and Alvin have a very competitive relationship. They are constantly at odds with one another and fight, but they do show that deep down they love each other. They are shown in the future as married with twin boys.
Brittany and Simon are friends, but they don't interact very much often. In episode "Simon For President". Brittany works with Simon running a class president to run against Alvin, soon after Brittany begins giving Simon advice, the next day, Simon discovers his posters defaced and tries to "play the game" but Brittany has to take it further for him.
Brittany and Theodore are friends, but they don't interact very much. She seems to care about Theodore such as in "Dragon Dad" when she did not want to take the dragon egg from Theodore, but did so not wanting him to be upset when it failed to hatch.
In an interview, Janice Karman explained the origin of the name "Brittany" stating "Well this was way before Britney Spears or any other Brittany I ever heard of. We had a singer in the studio and that was her name, so that’s why I named her Brittany because I liked the name."
Brittany is very similar to Alvin: They are the leaders of their respective groups, very competitive, enjoy extracurricular activities, often the most outspoken, stubborn, arrogant, selfish and conceited. Their unwise decisions or "spotlight" personalities commonly are the cause of many mishaps and adventures. Despite their selfish interests, they usually come through for their siblings in the end.
Brittany "hates" green (as stated in Cinderella? Cinderella!) and never really takes the time to realize her younger sister Eleanor wears (and likes) green. However, Eleanor doesn't seem to be offended by this statement. Though she could mean she doesn't look good in green.
She also hates black. In the 80s episode The Greatest Show-Offs on Earth, Alvin tells everyone to wear black to avoid being spotted, but Brittany refuses to and instead dresses in a pink superhero costume. In the CGI episode Albrittina, Brittany does admit she doesn't look bad in black.
Brittany is nyctophobic or afraid of the dark.
Brittany is the only member of the Chipettes who kept her original eye color in all versions.
In the CGI series, Brittany has been shown to be a talented mimic, able to disguise her voice, and even fake accents to fool people into thinking she is somebody else; she does a very convincing impersonation of Principal Meadows (in Saving Simon) and a rather exaggerated impersonation of Alvin.