Look around the internet for Nappa fanart. Or Nappa fanfiction. You're not likely to find much, and most of which you find is certain to be directed at the non-canonically Team Four Star version of the character. Who is Nappa? It sounds like a rhetorical question on a Dragon Ball blog; Obviously he's one of the four surviving Saiyans in the original manga, partner to Vegeta and Raditz. But that's not what I meant. Very little is known about Nappa, comparative to the other four Saiyans. Goku and Vegeta we understand and can infer many things about, due to their roles in the series. Raditz too is lent some credence above Nappa from nepotism. (Goku and Bardock nepotism, naturally.)
You could define Nappa, as they do over at Dragon Ball Encyclopedia, in the following ways: "Nappa has quite a destructive personality and loves demolition to an extreme level.", "Nappa also has a very cocky personality, as he enjoys toying with his opponents.", and "He tends to fight first and makes use of his brawn more so than his brain." Take some key words from those sentences. Destructive, cocky, brawn. This suggests a fairly one-dimensional character; Nappa, a Saiyan who loves to fight and win. But that doesn't sit right with me. It sounds too familiar.
What if I were to say that I feel Nappa is ultimately the most misunderstood character in the Dragon Ball franchise? You might think I'm being hyperbolic, but if you stick with me I think I can convince you otherwise. Even more, I'd like to make an argument that Nappa was the obvious choice to become the new ally to Goku and friends from the Saiyan Saga, as opposed to the nihilistic Vegeta.
Nappa is a Saiyan in every sense. He's powerful, competitive, and not too brainy. Of their duo, Vegeta takes charge of their strategies, leaving Nappa free to focus on fighting exclusively. Would their strategies be different if you took Vegeta out of the picture? It's almost a fact they would be. Take this exchange between the two Saiyans from Dragon Ball Z episode 5;
Vegeta: They mentioned something... interesting, didn't they? 'Dragon balls'... or something.
Nappa: Yeah. It sounds like they can grant any wish.
Vegeta: All right then. Let's go.
Nappa: We're going to bring Raditz back to life, right?
Vegeta: Don't make jokes! We don't need that good-for-nothing any more! We won't ever grow any older, but instead have eternal youth. How does that sound? If that were so, we could enjoy fighting forever!
Nappa: I get you. That would be great!
Nappa's initial instinct with his one wish would be to bring back his fallen comrade. Vegeta is the one who brushes off the idea as a joke, and talks Nappa into his own scheme by "we could enjoy fighting forever". Nappa, on his own, values fighting as much as the next Saiyan, but he's clearly loyal to his comrades, in a way Vegeta is demonstrably not. Let's have another look, this time from Dragon Ball Z episode 23;
Vegeta destroys the Saibaiman that was defeated by Tenshinhan.
Nappa: V-Vegeta, how could you do that? (commercial break) W-Why?
Vegeta: Anything more would be a waste of time. He couldn't win. That Saibaiman took him too lightly when he attacked. I told him to go at it with everything he's got.
The Earthlings are in awe of Vegeta's power, some discussion happens.
Nappa: (Talking to the Saibaimen) You guys, go all-out from the start this time!
Nappa is not only horrified by how Vegeta disposes of their ally, but takes it upon himself to coach the remaining Saibaimen, in an obvious effort to keep them alive. Vegeta has a complete disregard for the Saibaimen's, as much as Raditz's life. And lest we forget who eventually turned against Nappa.
Of course Nappa is no saint. He manages to, in the short amount of time he's in the story, inadvertently kill Chaozu, Tenshinhan, and Piccolo, and makes several solid attempts at killing Kuririn, Gohan, and Goku. But it is interesting how he never manages to kill who he's aiming for. This is a lot to do with luck and strategy and timing on our heroes' behalves, but some of it is to do with Nappa's own goals. He wants to fight, and he wants to enjoy it. And unlike Vegeta, he isn't compelled to step over or sacrifice his allies to do so.
Nappa is a Saiyan, and Saiyans are born with an incredible fighting instinct. Even Goku often puts everyone in danger because he can't help but be excited by a stronger opponent to challenge him. In that sense, Nappa and Goku are hardly different. Neither are particularly brainy, nor particularly malicious. In the society which Nappa grew up in, lower class warriors were already considered less-than-Saiyan, being little better than animals. So to an Elite Saiyan, the Earthlings can hardly seem like little more than animals. As far as Nappa might be concerned, exterminating the Earth is the same as any human hunter during deer season. He likes doing it, and the deer/Earthlings don't have as much "humanity" as him anyway.
Goku grew up in very different surroundings, by his own very good luck. Were Nappa to have been raised by Son Gohan and Muten Roshi, would he have been much different than Goku? I don't believe so. Unlike Vegeta, who seems to have manipulation etched into his very core (demonstrated by his actions throughout the series up until his sacrifice against Majin Buu), Nappa is merely along for the ride. Even as a hero, it takes Vegeta a long time to become a team player and behave altruistically. Nappa is primed for altruism almost from the moment we're introduced to him! "We're going to bring Raditz back to life, right?"
Son Goku is no stranger to having friends who once wished him harm. Bulma manipulated him into becoming her bodyguard so she could steal his Dragon Ball, Yamcha and Puar tried to rob and maim him, Kuririn tried to cheat him out of Muten Roshi's favor multiple times, Tenshinhan and Chaozu aimed to kill him under the orders of their teacher Tsuru Sen'nin, and Piccolo planned to kill him as revenge for his father Piccolo Daimao. All and all, that's a strange list of folks to call your best friends. Yet Goku does, and it's one of the best parts of Dragon Ball. Friendship and redemption.
Through a very long character arc spanning almost the entirety of DBZ, Vegeta makes this transformation from murderous rogue into Goku's close friend. But if we're looking at the Saiyan Saga, and Dragon Ball before it, and nothing that comes after it, Nappa seems infinitely more redeemable. Vegeta is cruel, manipulative, sociopathic, and self serving. Whereas Nappa, like Tenshinhan before him, is simply in the fight because it's where his destiny has lead him. Unlike Ten, Nappa doesn't get to fight Muten Roshi and learn that fighting without malice can be more rewarding than with it. Instead he is betrayed by the last ally he has, Vegeta.
Akira Toriyama is quoted with the following statement about Vegeta in Daizenshuu 2; "With Vegeta, well, I don't like him all that much, but he was extremely helpful to have around." (Translation via Kanzentai) It's no secret that fans love Vegeta, and Toriyama often kept characters around because of their popularity. He might have out-and-out confirmed this, but I speculate that Akira Toriyama kept Vegeta in the story purely based on fan response to him. (This isn't unreasonable. Why else would he keep a character around he didn't like? Of course, I could be woefully mistaken.) But had things gone differently with Vegeta's fate, Nappa's might have still stayed the same. Indeed, it's almost certain that Toriyama killed off Nappa without batting an eyelash. But what if he didn't?
In an alternate reality, I like to imagine that Nappa survived Vegeta's betrayal, and returned to the battle field after Vegeta was hit with the Genki Dama to lend them a hand in defeating him. Essentially, usurping the role of Oozaru Gohan, or at least drawing that section out slightly. Once Vegeta had left, I think Nappa would fall to his knees and beg forgiveness of Goku, Kuririn, Gohan, and Yajirobe, having seen the error of his ways through Vegeta's actions against him. From then on, I'd like to imagine he would attach himself to Goku as he was once attached to Vegeta.
(fanart via http://blog.livedoor.jp/shiki057/)
The Uncle Nappa theory is my own pet bit of "head canon", where Nappa was allowed to live and join the ranks of the Z-Senshi, through Goku. I see him growing into a fun loving soul, like Goku, but often more loyal to spending time with his new-found Earthling friends than Goku himself, as his own private redemption for his decades of service under Freeza. I see him fighting on Namek along with Goku, having taken the Capsule Corp. ship into space with him. I see him being wished back to Earth with the others, and living with Gohan and Chichi while they wait for Goku to return from space. He would fight against Vegeta when the latter would allow Cell to attain his Perfect Form, and would stand up to him with Trunks after he defeats Vegeta.
And I hope that, given my argument for the redeeming qualities of Nappa, you too will join me in lamenting this potentially great character.
Rest in peace, soldier.