Character sprites are selected by job. If a specific sprite exists for the character's job, that one is chosen. If not, the game checks to see if a sprite exists for the character's job category. If neither of these sprites are found then the default Citizen sprite is used.
I think there's a lack of consistency in the sprites which exist now. Some of them are hand drawn, others are 3D renderings. Their height varies too much. Many of them are just plain ugly.
No Specified Job- The Citizen

This is the default sprite. I think the pants color should be changed to red or dark grey.
ACADE- Scientists, Teachers, ProfessorsNo sprite.
ADVEN- Bounty Hunters, Arena Pilots, Explorers

Sonata can be seen wearing this armor type in the GearHead comic:
http://ataraxiatheatre.com/2009/11/25/toast/CORPO- Corporate Executives, other office workers
Edit- I forgot that Phil did this male corporate sprite. That's what I get for posting right before bed. No female sprite.
CRAFT- Mechanics, Engineers, Carpenters
FAITH- Monks, Warrior Monks, Priests
LABOR- Truckers, Miners, Factory Workers
This sprite is based on the character Tama from the GearHead comic. No male sprite currently exists.
MEDIA- Singers, Reporters, Pro AthletesNo sprite.
MEDIC- Doctors, Nurses, Paramedics
MILIT- Soldiers, Mecha Pilots
POLIT- Politicians, Bureaucrats, LawyersNo sprite.
POLIC- Police Officers, Forensic Investigators

This image is based on the Guardians from GH1, who are a rough-and-tumble Judge Dread-style law enforcement faction. A different image is needed for the officers of L5 Law.
THIEF- Bandits, Thieves, Hackers, Pirates, Street Punks

Several characters can be seen wearing this armor in the GearHead comic. Gronda is based on the female bandit sprite.
http://ataraxiatheatre.com/2009/07/10/bar-brawl-of-tragedy/TRADE- Shopkeepers, Traders, Venture Capitalists
MOOK_- Bad GuysThis is a special job type which sometimes gets applied to enemy henchmen. No sprite.