Tilly is a very unusual girl. She tends to have a very quiet demeanor, but hidden underneath is a creative, imaginative and sometimes surreal child. Tilly is very kind to those around her and much like her younger brother Cricket is willing to help those in need. While she seems to be unintelligent at times, she actually houses a very wise and open mind.
Tilly is shown to be somewhat controlling at times. In "Mama Bird", she insisted on being a trio of baby birds' mama despite Bill telling her not to interact with them and their fascination with Cricket rather than her. She also attempted to scare Cricket from doing dangerous stunts in "Cricketsitter" only to accidentally dislocate his arm and fretted about being punished by Bill. In response, she shows a very determined side to set things right.
Tilly is a very small girl and only slightly taller than Cricket. While she displays a yellow complexion like her family, she is noticeably more pale than the rest of them. She has short black hair and freckles underneath her eyes. Tilly also has buck teeth similar to Cricket and full cheeks. She dresses in a light violet dress and unlike her brother, actually wears shoes.
She bares a resemblance to her great great aunt Metrona Green.
- Tilly bears a striking resemblance to Prudence from Cha Ching.
- Tilly apparently has a habit of personifying objects which seem to reveal a darker side to her.
- Tilly seems to have the ability to befriend any creature regardless of species, with the exception of the Patagonian Python from "Cheap Snake" after she began to hear sinister and apocalyptic noises coming from it.. In "Tilly Tour", she befriended an unusual creature called a "Critagion" which is a combination of a rat, cat, pigeon and "who knows what else."
- Tilly apparently sleeps during the day, which Cricket warns her against.
- According to Chris Houghton, Tilly has named every single animal that the Green Family owns and have met. However, she is apparently the only one who actually remembers their names.
- Tilly is convinced that she is actually a squirrel, something that she has only confided in with Cricket and Andromeda.
- In a Tumblr Q&A, Chris Houghton joked that the reason why Tilly's hair is black, even though the rest of the family does not have black hair, is because she wears a wig.