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 also repeatedly moving his lab so that the risk of someone finding it lowers.
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.