He shows the occasional glimpse of humanity, however, and other characters have occasionally warmed to him; Lisa dates him briefly in "Lisa's Date With Density", Marge informally adopts him in "Sleeping with the Enemy", and Bart befriends him in "The Haw-Hawed Couple".
Even in a life-or-death situation, Nelson laughs at the one in danger. Often, he makes his victim inflict self-punishment while yelling remarks like "Stop hitting yourself!", "Stop butlering yourself!", "Stop zapping yourself!", or "Stop endangering yourself!". Usually it is Milhouse and Martin who are subjected to this type of cruelty, although Nelson once described Bart as having "spiky hair, soft kidneys, and always hitting himself".
His family life is dysfunctional. While some early episodes offer inconsistent histories the canon story that has evolved is that he lives in a dilapidated house with his mother who works on the fringes of the sex industry, either as a waitress at Hooter's or in a topless bar. His mother seems to care for him but her character seems compromised by either alcoholism or disability (in one episode it is explained that a "carnival ride" hit her on the head. His father is said to have left the family or "went nuts." The character design and voice portrayal of both parents has varied throughout the course of the series.
Nelson's father appeared to have left after apparently going to the store to buy cigarettes, but in the fourth season episode, "Brother from the Same Planet", he appears as the children's soccer coach and awards Nelson with a free trip to Pele's Soccer and Acting School. Nelson's father returns in the 16th season episode, "Sleeping with the Enemy". It turns out that he did not leave Nelson deliberately; he bit into a chocolate bar, not knowing it had peanuts and had an allergic reaction. Looking like the Elephant Man, he ran out of the store and encountered a circus that made him a part of its freak show. At performances, circus attendees threw peanuts at him, which perpetuated his reaction, never allowing him to return to normal. When the circus came through Springfield, Bart recognized him and brought him home to rid himself of Nelson (who had been taken into the Simpson home by Marge). Judge Muntz is his grandfather.
In one episode, Nelson represents Japan at the school's Model UN club. In various episodes, he gives off the impression that he is a lot smarter than he may first appear. Nelson often points out painfully obvious things to adults and kids alike that take them longer to grasp. Another example is a running joke where Nelson does a class assignment which is implied to be of high quality. However, he is always brushed off by the teachers before he gets to show them his work. On the other hand, he does often demonstrate stupidity. When Principal Skinner tricks Bart and the bullies by telling them they are getting bikes but instead locks them in the closet, Nelson still believes he is going to get a mountain bike but that may be more blind trust than true stupidity as several times Nelson has chosen a fantasy to believe rather than the truth that he could figure out with a little thought.
Nelson is also a natural athlete. In the episode "Bart Star", Nelson almost singlehandedly carries the entire Springfield Pee-Wee football team. As the team quarterback, he also on one occasion catches his own pass and plows through the opposing team with extreme ease. Nelson is also a fan of many unexpected things, such as Andy Williams and Snow White. He is also an expert in the proper preparation of huckleberries (also probably a reference to Nelson's similarities to Huckleberry Finn, whom he portrayed in the episode, "Simpsons Tall Tales"). According to the episode, "The Monkey Suit", Nelson is apparently a master of disguise, fooling even Bart and Ned Flanders.
Although Nelson is still as violent as ever, he has also been known to beat people up for arguably noble reasons (usually it is Bart who is subjected to this type of punishment). In one episode, he punched Bart in the stomach, saying "That's for taking credit for other people's work!", when Bart (truthfully) claimed to have been Krusty the Clown's assistant or when Bart wrecked Hitler's vehicle at a carnival ("You wrecked Hitler's car! What did he ever do to you?!"). He also beat up Bart for "Wasting teacher's valuable time!", when Bart constantly raised his hand with the wrong answer to answer a question Mrs. Krabappel asked, and "Besmirching an innocent girl's name!" when Bart tried to flaunt his relationship with Jessica Lovejoy to the other kids. However, in The Simpsons Movie, when Milhouse claims to care about the environment in order to impress Lisa, Nelson stands over him with a fist raised and demands he "Say global warming is a myth!" Milhouse does so, but mentions that further study is needed, and Nelson punches him telling him "That's for selling out your beliefs!" Nelson himself was subjected to a beating at the hands of Kearney and Jimbo at the racetrack when he used the word "guff" instead of shit on the Season 11 episode "Saddlesore Galactica".
In the episode "Little Big Girl", it is revealed that Nelson is of German descent. However, ironically, in the episode "Much Apu About Nothing", Nelson picks on Üter Zörker for being German. At times Nelson is depicted as having master answer sheets to various tests and assignments at school. In one episode he has an "office" located in a toilet stall where he has the answer sheets hidden in the toilet cistern. He also has a framed picture of himself mounted on the cubicle wall with "Employee Of The Month" stenciled below it. Nelson's most embarrassing moment was in the episode "22 Short Films About Springfield", when he was forced to march down the street with his pants pulled all the way down to his ankles to a jeering crowd for his years of torment, while Bart and Milhouse squirt ketchup and mustard on him from a highway overpass.
In the earliest episodes, Lisa became the target of Nelson's bullying, which prompted Bart to defend her. Bart's actions resulted in frequent beatings by Nelson and his gang, but finally ended when Bart received help from Herman, who came up with the idea of standing up to Nelson and his gang by pelting them with water balloons. From that point on, Nelson shows a grudging respect for Bart, though he occasionally reverts back to his past behavior towards him.
Although still prone to violence, Nelson occasionally hangs out with Bart and his outcast friends such as Milhouse Van Houten and even common bullying target Martin Prince. In a recent episode, it is shown that Nelson has become Bart's "other best friend", along with Milhouse.
Nelson has many other moments where he displays his hidden good nature, such as his brief amorous relationship with Lisa. Although he cannot control his delinquent tendencies, he treats her with respect and even tries to change for her, although both of them realize that he is not being true to himself by doing so. In "Lisa's Date With Density", Nelson kisses Lisa, only to be berated by Jimbo, Kearney, and Dolph, who believe that kissing girls is "gay".
He also punishes Sherri and Terri for tormenting Lisa. Another curious example of his "good side" is with Martin, a boy whom Nelson picks on perhaps more so than any other kid in town. Despite the utter cruelty, there are occasional hints that Nelson does not harbor any real hatred towards Martin, and does not want to pick on him. In the episode, "I'm Spelling as Fast as I Can", both Nelson and Martin attend Space Camp and it is implied that Martin had helped Nelson quite a bit and the two may have been friendly. Due to the peer pressure of maintaining his bad-boy image at school, however, he seems to deliberately go back to his antagonism toward a rather dismayed Martin. In "Dial 'N' for Nerder", when it is believed that Martin has died, Nelson seeks out to learn the truth and find Martin's killers. When it is revealed that Martin is alive, Nelson mocks and punches him, but also states that he is glad Martin is not dead.
Ironically, in "Bart to the Future" Nelson is shown to be a rich, powerful businessman or mob boss, who owns a bar.