- For a list of comics, see Comics featuring Jill.
Jill is a stick figure character in xkcd. She is usually distinguished by being clearly a child, both in terms of her behavior or in comparison to the size of the adults around her. Her hair is typically set in a hair bun but compared to Hairbun it is looser and has a string of hair hanging down her back from one or sometimes two hair buns. As a rule of thumb, if a woman with a bun is not clearly a child, she's Hairbun and not Jill.
Her name comes from 1662: Jack and Jill, the only comic where she is given a name. In the comic, she represents Jill in the traditional "Jack and Jill" English nursery rhyme. Up until a few years after the comic was posted, she was generally known as Science Girl because of her commonly exhibited personality and interests.
Jill is a minor character in xkcd, but there are several comics where she is the main protagonist. She has a marked interest in science and is typically more knowledgeable about the subject at hand than the adults around her, which may lead to embarrassment, enlightenment or in some cases dangerous situations, depending on how obtuse or condescending the adults around her behave.
She is not necessarily the same character from comic to comic, but she does represent the general kid interested in science and intellectual rigor. In 1860: Communicating, she appears as Alice in Wonderland and takes advantage of Humpty Dumpty's famously illogical dicta on linguistic pragmatics. In this and other examples, her attitude often makes people uncomfortable, especially adults. These personality features distinguish her from an everywoman character like Hairbun, but although Jill often causes consternation, she not generically evil like Black Hat, and thus in no way would she represent a young version of Danish.
One of her largest parts is in 1611: Baking Soda and Vinegar, which is one of those that becomes dangerous for the obtuse adults. She is also ready to commit crime, even though she knows about environmental issues in 1659: Tire Swing (the only comic with her so far where there are no adults, but it is clear that she is a girl), where she, along with another girl, steal the tires off a man's car, and then fight him. But she may also just induce new interest for science in Megan as in 1104: Feathers, or just display general interest in science as in 1352: Cosmologist on a Tire Swing or the first comic (found so far) with her 585: Outreach (where she only had a ponytail). It was also in this first appearance we learn why she wants to become a scientist.