b. A shady character.
c. Someone that appears to have no moral judgment, self-respect, or general hygiene to the point where he/she is a total laughingstock that must be avoided.
She dumped him and went out with two other guys on the same day? Holy sketchball, batman!
"That dealer is a sketch ball to the max."
b.) Someone who participates in activities that you wouldn't want the majority of people to know about because people will think you're creepy, and avoid you.
Normal behavior of a sketchball include:
Hitting on girls outside of preppy suburban bars.
Sitting by themselves at night on their sidekick/blackberry
"Accidently" sleeping with people
Lurking people like crazy on facebook/myspace/AIM.