In The Great Gatsby, Nick Carraway is the honest, reliable narrator. Nick opens the story by informing the reader that he has been taught by his father to "reserve all judgments." Nick is therefore an objective, reliable narrator (and character) and it is through his eyes that we see the other characters in the novel.
Tom Buchanan is essentially "the bad guy." He comes from a wealthy family, entertains racists ideologies, cheats on his wife and mistreats people in general.
Daisy Buchanan is one of Fitzgerald's "it girls," those girls who are beautiful, tending to come from money and therefore the object of most men's desires. Daisy does have a genuine nostalgic love for Gatsby, but above all, she represents money. When she hits Myrtle with the car, she retreats to Tom. So, there is something tragic but also superficial about Daisy; she finds herself married to a philanderer but embraces the lifestyle he has given her. Jordan Baker is also dishonest, a typical good-looking socialite.
Tom might be the foil "bad guy" to the "good guy" Nick. Gatsby, on the other hand is somewhat of a paradox. He is a genuine romantic, an idealist. Gatsby represents the American dream, a rags to riches story. He does all he can to win Daisy, as if she herself was the Holy Grail at the end of his American dream adventure. But, in order to put himself in the social position in order to even be near Daisy, he must create a secondary persona. Gatsby pursues Daisy, partly out of love and partly out of an idealization of her. His affection for her is genuine and his desire to pursue her is all encompassing. But the fact that he created a different persona and that he entertains high society parties makes a part of himself dishonest; not to mention, he has illegal dealings which led to his wealth.
One note about the overall characterization in The Great Gatsby is that it is a portrait of honesty/dishonesty. Of all the characters in the novel, the one our honest, reliable narrator identifies with the most is Gatsby. Despite the fact that Gatsby is somewhat "fake," a created persona, he is more genuine and unselfish than any other character (other than Nick and Mr. Wilson). This is what makes Gatsby so endearing. He is not a calculating dishonest man like Tom. He is not as superficial as the socialites he hosts at his parties. He only does dishonest things in order to be close to Daisy. Through Nick's eyes, we root for Gatsby despite his flaws because his intentions and motivations are based on his naive idealism.
This characterization also illustrates the differences between appearance and reality as it applies to the theme of honesty/dishonesty.