Thesaurus lacks the sweet, light-hearted nature of WordGirl in favor of a more jaded outlook on life. He generally assumes people aren't as intelligent as him, doesn't enjoy educating others on anything he deems basic (but will jump at the chance to look smart nonetheless), and acts rather introverted. Much of his time is spent on his research, which can encompas anything that he happens to find interesting that day.
Surprisingly, his introverted nature isn't simply a desire to be left alone, but an inherent shyness that keeps him hiding in dark corners or only coming out at night. He's not very good with "the big city" and finds himself overwhelmed easily, or following along others who offer him help without much question. While not guilible, per say, he does seem to understand that the average Fair City citizen knows more about the area and culture than he does.
He's not really "evil" in the traditional sense, but does steal money on occasion. Thesaurus generally targets ATMs and other automated machines, simply to avoid causing a ruckus. He rarely damages property, and even more rarely lashes out against people, including WordGirl. Most of his crimes center around the coveting of information, stealing shipments of books, holding libraries hostage, or aquiring rare and antique texts. He scans books into his supercomputer to archive the information, and it's generally known that if he were allowed to have free reign of a library for a week, he would leave it in pristine condition when finished. Besides stealing rare copies of books and keeping people out of the building for several days, he's relatively harmless.
Thesaurus prefers to talk his way through things, and only lifts a hand to fight when sufficiently prompted. This can be a difficult task, however, as Thesaurus is quite good at talking, and will talk circles around others to avoid physical confrontation. He'll often throw in particularly large words or complicated topics to ensure his superiority.
Thesaurus is afraid of a few things, namely: spiders, dogs, sheds (because they could have spiders), and The Easter Bunny. He'll generally avoid these things or flee from them outright, usually abandoning whatever he was after in the process.