Update!: Found a whole bunch of twitter accounts I missed. Most of the cast of Final Fantasy 7 is on here in character!

Looks like EVERYONE is on twitter now. Even video game characters. The best part is these stay in character and regularly update.

Active Players
This is the active crowd, the real players. People that don't break character and have fun being their video game heroes.
Chris Redfield - Resident Evil
Ninten - Earthbound 0
Pyro - Team Fortress 2
Heavy - Team Fortress 2
M. Bison - Street Fighter
Sam - Sam & Max
Max - Sam & Max
Prince Of All Cosmos - Katamari Damacy
Dr. Wily - Megaman
Updated:
Sephiroth
Barret
Aeris
Cloud
Red XIII
Tifa
Cait Sith
Cid

Pokémon
In addition to video game characters there is also a large Pokémon presence on Twitter! Usually staying in character these guys even have their own twitter directory to keep track of the players!

Not Active
These are some characters from the past that used to be active twitterers but now are letting their accounts idle while they set off on adventures!
Mario - Super Mario Bros.
Luigi - Super Mario Bros.
Captain Olimar - Pikmin
Master Chief - Halo
Waddledee - Kirby

If I missed you or you know someone that runs a video game character Twitter please leave a comment and let me know the address and I'll add it to the directory!