Dexter is the name of Ash Ketchum's Pokedex. The Pokedex was invented by Professor Oak and provides information about Pokemon such as basic descriptions, moves and type. Over the course of the series Dexter has received various upgrades and models to constantly adapt to new Pokemon discoveries and regions.
In the English dub version of the Pokemon anime series, Dexter was first voiced by 4Kids voice actor Eric Stuart for the Kanto, Orange Islands and Johto sagas.
For the Hoenn saga Dexter had a voice change and was voiced by 4Kids voice actor Rachael Lillis.
For the Battle Frontier saga Eric Stuart once again voiced Dexter until Pokemon USA took control of the anime. Dexter was then voiced by Bill Rogers.
For the Sinnoh saga Dexter had another voice change and is voiced by Michele Knotz.
For the Unova saga (Besh Wishes) Dexter is voiced by Marc Thompson.