Thea Stilton herself.
|First appearance||Lost Treasure of the Emerald Eye|
|Residence||New Mouse City|
She is gray unlike Geronimo, she has purple eyes and always wear a coat and a pink shirt and blue pants.
She appears in almost all the Geronimo Stilton books, sometimes briefly. She always appear in the Thea Sister books.
- Thea's fur is gray unlike Geronimo's which his fur is brown.
- She is a adventure mouse unlike Geronimo.
- ↑ Since Thea refers Geronimo as "Big Brother" sometimes,she must be his younger sister