|First appearance||Lost Treasure of the Emerald Eye|
|Residence||New Mouse City|
Geronimo Stilton or OOG (code name) is the main protagonist mouse that appears in all of the books. he is the editor of "The Rodent's Gazette". He is brainy but a scaredy-mouse as he says in the books. He has several friends and has a sister name Thea, two cousins named Trap and Dino, a nephew named Benjamin,a grandfather named William and more. He likes Cheesy Chews. He is the main character of the series. He is also a secret agent.
- He appears in all the books.
Geronimo is a mouse with big ears and a quite small nose. His body color is brown with patches of white under his feet and on his chest continuing up to his mouth. He mainly wears a dark-green suit with a red tie, a light-green buttoned shirt underneath with lighter green trousers and glasses.
- He appeared differently in The Wild, Wild West and the future books.
- Most of the time he is wearing his uniform.
- He is a scaredy-mouse.
- He doesn't like adventuring.
- He doesn't like traveling.
- He does not know how to tell jokes.
- He has a crush on Petunia Pretty Paws.