Professor von Volt.
He is a Professor who goes everywhere on trips, or to perform experiments too dangerous(at least to some) for other mice to be around.Nevertheless,he's a kind rodent,and a recurring character.
He is in some of Geronimo's adventures. He is also Geronimo's friend. He is in :
I'm Too Fond Of My fur!
Geronimo and the gold medal mystery
The temple of the ruby of fire
the phantom of the subway
Geronimo Stilton: The journey through time.
Geronimo Stilton: Back In time, the second journey through time.
Geronimo Stilton: The race against time, the third journey through time.