Mabel Pines is a 12-year-old girl and is the deuteragonist of the series. Alongside her brother Dipper, she spends the summer of 2012 with her Great Uncle Stan in Gravity Falls, Oregon, where she and her brother frequently encounter the supernatural. She takes a much less serious approach to life than her twin brother Dipper, as she navigates her way around odd, new surroundings. Mabel is an energetic, fun, bouncy, hyperactive, enthusiastic, and free-spirited girl. She is an optimist who skips through life with a braces-filled smile and an assortment of extremely colorful knit sweaters. It is Mabel's outgoing personality and fearless curiosity that comes in handy when Dipper needs help solving a mystery, even if she isn't always aware of her silliness. She shows a more sophisticated attitude when it comes to art. Despite all the weirdness in Gravity Falls, Mabel is always a "glass-half-full" kind of girl. She is rarely angry or upset, and generally keeps a positive outlook. Despite her cheerful attitude, Mabel is shown to be quite sensitive, as she can occasionally get her feelings hurt, especially by her rival, Pacifica Northwest. She is somewhat naive at times. She avoids lying and is a terrible liar, often blurting out whatever it is about which she is trying to lie. Mabel is aware of her charm and cherishes her cuteness and beauty. She generally has a high opinion of herself, considering herself "irresistible" to boys, claiming that she does everything perfectly, and fully believing she is of "pure, perfect heart". Mabel has also on occasion made mistakes that accidentally backfire in some way. However, like her brother, Mabel has the best of intentions and she will always try to choose the right options for the happiness of herself and others, she also proves to be clumsy.

Mabel is a preteen dreamer with a world filled with preteen novels (mostly about vampires), and her fixation on finding her one true love occasionally lands her in sticky situations. She sometimes has her moments of self-centered behavior, most notably when it comes to boys with whom she's infatuated with.