Goku is a kind and good natured person, who is peace loving by nature, but loves the challenge of a good fight. He is always striving to be the best, and usually suceeds! He is selfless when it comes to protecting others, and has sacrificed his life on more than one occasion. He is the hero and main character of Dragonball Z up until when he dies for the second time. By the end of the show, Goku is the most powerful being in the Universe, is around 45 years old, and is evena grandfather! He's come a long way from the innocent little boy we saw in Dragonball.

Gohan

Gohan is Goku's son. He is half Saiyan, half Human. This means that he has the potential to surpass everyone in the Universe in terms of power. However, unlike his father, he hates fighting; and only ever uses his true power when he is angry. Like his father, he is gentle and polite. He hates the idea of killing anything. Gohan prefers studying to fighting, although this may be to please his mother more than his own choice!

Krillen

Krillin is Goku's best friend. He is short, bald, and has six dots on his head. He is a fighter, like Goku. He is less soft hearted than Goku and Gohan, and is willing to kill people if he has to (although Goku has killed 2 people out of necesstity). He would like to have a full head of hair, as he has never had any. Krillin and Goku were both trained by Master Roshi when they were children, and they were always competing, but Goku surpassed Krillin over time, due to the fact that he was a Saiyan.

Piccolo

When an alien from the planet Nameck decided he wanted to be God of the Earth, he first had to purge himself of all evil and hate. His spirit was split into two, the good side was called Kami-sama, and he became God; the evil side was called Piccolo Diamou. When Goku killed Piccolo Diamou, the evil demon spat out an egg with his final breath. This egg hatched into Piccolo. Piccolo, like his father, was evil. He grew up wanting to kill Goku, and hatched many an evil plot to succeed, and to take over the world. Piccolo was Goku's arch-enemy in Dragon Ball. However, when he did finally kill Goku, accidentally, while killing Raditz, he looked at Goku's lifeless body, and decided that that wasn't what he wanted. Piccolo has become less evil, and has almost become a good person. He at least has some good in his heart, and he protects Gohan at all costs, even taking a fatal blow for him.

Vegita

Vegita is essentaially evil. He was raised by the evil Frieza, as he was the most powerful Saiyan the world had ever seen. He also happened to be the Crown Prince of the planet Vegita. When he met Goku, he couldn't believe that there was another Sayian that could match his power; and even surpass it. His main goal in life therefore is to defeat Goku, and prove that he is the best. All this while maintaing an uneasy alliance with Goku. His will to live is taken away from him when Goku is killed for the second time, and there is no way to bring him back. His one objective in life has been taken away from him. He does eventually develop a consience, and when he starts a family, he learns to love people other than himself.

Bulma

Bulma is the daughter of Engineering Genius Dr. Brief, and shares his amazing skills for inventing and building things. She happens to be extremely short tempered though, and when things start to go wrong; she likes to vent her feelings. She is a good person however, and she cares very much for Goku and Gohan. She is the former sweetheart of Yamcha, but she ends up marrying... VEGITA!! Yes, it's true! They have two children together: Trunks, and Bra (many people are named after underwear in Dragonball Z). She is a ggod mother, and dotes on her children, as she is more able to give them affection than Vegita is.

Other notable characters are:

Yamcha, Tien, Choutzu, King Kai, Master Roshi, Chi-Chi, Ox-King, Kami-sama, Mr. Satan, Videl, Goten, and Trunks.
However, I don't have enough information about them to make a character profile of them.