Thea Stilton is the biological sister of Geronimo Stilton and is the only member of the Stilton family who is biologically related to him in any way, as the two were adopted by the Stiltons and never met their biological parents (needs clarification). Raised presumably by William Shortpaws, the two were taken into the Stilton family and adopted the family name.

Thea was most likely top of her class in New Mouse City, because she was eventually invited to attend college in the prestigious Mouseford Academy, and she graduated with top honors in her batch. After graduating, she went on to work as a well-known journalist in the Rodent's Gazette and is currently the newspaper's special correspondent.

Sometime down the line, she was invited back to Mouseford Academy by headmaster Octavius de Mousus as an investigative journalism teacher, and there she meets five girls that she eventually takes in as her students and proteges. She writes about these girls' adventures around the world in a style similar to her brother Geronimo's books