Radditz

The first villian of Dragon Ball Z, Radditz is Goku's brother. He comes to Earth to check on Goku's progress with exterminating all life on the planet. He doesn't live very long, as Goku and Piccolo put a stop to him after he kidnaps Gohan. He reveals Goku's past to him when he first meets him, telling him about his real name, Kakarot, and how he is a Saiyan.

Nappa

Nappa is a large Saiyan with a small brain. He doesn't have much of a past that is revealed to us, but appears to be Vegeta's bodygaurd and came along with him to check on things on Earth. He is originally stronger than all of the Z warriors, and even kills Piccolo, but then is beat up by Goku and killed by Vegeta in discrace.

Vegeta

Vegeta is the Prince of the Saiyans. He is King Vegeta's only son, and seeks revenge with Freeza throughout his early life for killing his father. After Radditz dies, him and Nappa head to Earth to deal with Goku. Vegeta is evenly matched with Goku but is eventually defeated. On Namek, he slowly becomes allies with Krillin and Gohan, since he can't defeat Freeza on his own. Before Namek is destroyed, he is transported with everyone else back to Earth, where he lives at Capsule Corp, and eventually befriends the humans. From then on, he helps the Z warriors.

Freeza

Freeza considers himself the strongest warrior in the galaxy, and seeks out the Dragon Balls on namek to make himself immortal. He is able to transform into many different forms, growing in strength each time. He is not hesitant to kill people who get in his way, and slaughters the Namekians who are hiding the Dragon Balls. He is defeated by Super Saiyan Goku, but is brought back to life by his father, King Cold, only to be defeated again by Future Trunks.