Nappa is a tall and muscular man with a thin mustache. Despite his appearance of being in his early 30s, Nappa is actually roughly 50. He is one of the larger characters in terms of muscle mass. When younger, he had black hair, but he has since gone bald.
Nappa is an arrogant, heartless, manipulative, obstreperous, egotistical, cocky, and violent brute who loves beating up anyone weaker than him. He is capable of being friendly, but like most other Saiyans, he is also very supercilious, persuasive, and hotheaded, with Raditz as a prime example.
It is shown that Nappa can be loyal, generous, and trustworthy to other Saiyans. He was extremely dependable to Vegeta, and though he did not personally like Raditz, he still suggested bringing him back to life. Also, in an anime-only filler, Nappa was very depressed about the death of the many Saiyans. He was likewise shocked to hear that Vegeta mentioned that every Saiyan deserved to die just for not being half as psychically strong as himself. Both Nappa (and Raditz of all people) were also disgusted to hear that Vegeta was willing to serve Freeza after killing his father.
When faced with the prospect of losing to someone he considers a weakling, Nappa will let blind rage overwhelm his thought processing. With Vegeta having to regularly remind (or in server cases order) him to calm down.