Nappa is Vegeta's sidekick who came to help destroy the Earth. He was also Vegeta's caretaker as a child, and is now very fearful of him. Nappa brings the Saibamen to help fight the Z fighters, and does much of the early fighting himself.
Nappa may have had more brawn than brain and tended to get overexcited and careless when fighting. Nappa did manage to eliminate Tien, Chiaotzu, and Piccolo before Goku arrived back on Earth. Goku stepped in just in time to save Gohan, and kill Nappa.