The bad guys are obvious to use because we're the audience, but not to the characters. The majority of them were Disney villains that most of use would be aware of, but no one else until they'd actually do something bad.
Aside from them would be Master Xehanort, but Terra really had no real to distrust him. Master Xehanort was a well known member of the community who Master Eraqus regarded as a brother. He was also the only one supporting Terra, even if only it was pretend.