Obviously Nova. Well Nova is fairly complex. By the end of the Novel she becomes like any other ghost albeit more powerful. She becomes brainwashed by the Terran Dominion Ghost program. A cold killer who would obey anything Mengsk ordered.

However before she was brainwashed she was pretty screwed up. She witness her family murder. She learns her sister was the one who planned it. Then she becomes a mind reading puppet for a gang in Tarsonis.

The other potential character I had in mind was Agent Kelerchian. A good cop who basically wanted justice served. However he tried to take down someone from the "Old Families"(the ruling class of the Confederacy). So he becomes a wrangler, someone who hunts for psychics to add to the Ghost program. By the end of the book Mengsk sends him to a suicide mission becouse he seems to not be 100% loyal to Mengsk.