If I had to compare her to anything, it would be like if Tintin listened to Le Tigre and joined theFantastic Four. She's excitable, passionate, attentive, curious, and caring. Like Captain Marvel, she's just a really powerful kid.
Her physical description usually sticks along these lines in most artist's usage of her:
She has short, dark hair. She usually wears aviation goggles on top of her head and a scarf around her neck. Otherwise, she dresses in comfortable clothes. She is average size and has a good body image. She has loads of confidence and charisma. She appears to be Asian or Native American. She has a ready smile.
For more information about Jenny Everywhere, see The Shifter Archive.
The following are some of my favorite renditions of Jenny, mostly found on the Jenny Everywhere Day tumblr site.