Dan "Lonely Boy" Humphrey is the son of rocker Rufus Humphrey, and is the older sibling of Jenny Humphrey. Dan is a poet and attends the St. Judes School for boys with enemy Chuck Bass. He has longed for Serena van der Woodsen since a party when they were fifteen, and she was the only person who was nice to him. He has somewhat of a disgust for Serenas best friend, Blair Waldorf; though, he seems to have warmed up to her given their heart-to-heart in Bad News Blair. Dan and his family reside in Williamsburg, Brooklyn, and he wishes to attend Dartmouth College.
Dan has liked Serena since they were 15 when Serena was the only person who spoke to him at a party, but he's never really spoken to her before. He lives with his father, Rufus Humphrey, and younger sister, Jenny Humphrey, in Brooklyn, New York. His mother lives away from home, in Hudson, as an artist. He is financially strained compared to his wealthy classmates at St. Judes, such as Chuck Bass, the party boy whom Dan hates the most.
Dan was one of the first people to see Serena back in person. He accidentally bumped into her causing her to fall and spill the contents of her purse, leaving behind her cellphone. In an attempt to see her again, he returns the phone to The New York Palace Hotel where she lives. He runs into Serena there and ends up on a date that with her so she wouldn't have to go to the "Kiss On The Lips" party which Blair is throwing. They have a rocky first date, going to his dad's concert. The date is ended early because he gets a text from Jenny saying that she needed help at a party. Serena came along and helped save Jenny from Chuck. The next morning Dan was freaking out over his embarrassing good bye, a wave. They end up going out again but he leaves because Blair told him about Serena sleeping with Nate, while he was dating Blair. They make up and become a constant source of attention.
At the Ivy Mixer, Blair tells the whole school and Ivy scouts that Serena is a patient at the Ostroff Centre, presumably for alcoholism. Serena then goes on stage and lies to protect the real patient at the Centre: her brother. Dan is surprised at Serena's apparent problem, especially after it seems that Serena has been making a real effort to change from her old partying ways. However, Jenny Humphrey reveals that Eric is the actual patient, which then causes Dan to respect Serena for protecting her family as he says that it was "cool" what she did up there on stage. This event actually strengthens their relationship, instead of weakening it like Blair had intended it to do.
Behind the scenes, though, Dan's childhood best friend Vanessa is there, who is clearly jealous of Serena and very much in love with Dan. Dan is oblivious to this. Despite Dan's clear preference for Serena, Serena can't help but feel jealous of how close Dan is with Vanessa, such as when Dan tells Vanessa that his mother came home, while Serena learns this from Vanessa instead of Dan himself.
When he learns from Gossip Girl that Serena was seen buying pregnancy tests he freaks out knowing that the child could very well be his as they've slept together. His father sees this as well and is extremely upset that his son may be a teenage father. Dan explains he will stay with Serena no matter what she does because he loves her. He tells this to Serena who says she's not having a child. Later on, he tells her that he loves her, and she responds "OK" which is not the response he was looking for. She later asks him why he loves her and he tells her a list he made with the reasons. She believes him and says she loves him as well, but she needs to go to help Blair. When he asks why, she says "one of the many reasons you love me."
Upon Georgina's arrival in the Upper East Side, she tries to get to Serena as much as possible. One of these ways is by getting to know Dan behind her back. Dan meets Georgina when her dog runs up to him. She announces herself as Sarah. It is later seen that Dan and Vanessa have taken "Sarah" under their wing as they believe she is a new girl to the Upper East Side who doesn't know her way around.
Eventually, Dan breaks up with Serena, because he believes that she has slept with someone else and he is tired of her holding secrets from him. A few hours after he breaks up with Serena, he kisses Georgina, believing that she is a helpless girl who is running away from a psychotic ex-boyfriend. Soon thereafter, Dan is convinced that Georgina is lying to him by being told the truth by Serena, as something like that "doesn't roll off the tongue". He and Blair manage to fool Georgina into thinking that Dan would like to meet up with her. The real means of the plan, though, was to get Georgina to meet with her parents and then get sent to a reform school. Dan and Serena rekindle what is left of their relationship before Dan ultimately ends it entirely. He is last seen spending some time with Vanessa during the start of the summer holidays.
 Season 2
According to Gossip Girl, over the summer Dan went from "Lonely Boy to Playboy". Dan occupied himself by having continuous dates throughout the summer, with Serena still on his mind. Due to Dan's emotional complications, he is unable to complete an essay he was to submit. This ultimately results in his being fired due to his lack of commitment. Upon being fired from his summer job, he heads to the Hamptons upon realising that he has been missing Serena. He hopes that by seeing Serena he will realise that they made the right decision, and he can move on.
He arrives in the Hamptons, at the annual White Party, to find Serena and Nate in a passionate kiss. This reveals his true feelings for Serena as he shows obvious anger towards Serena's "cheating". Serena tries to convince him that she kissed Nate so as to make an anonymous woman [to Dan] jealous. Dan goes to leave, when he is confronted by a pair of girls he had dated throughout the summer. The girls, having found out about one another's relationships with Dan, pour their drinks on Dan and leave. Having been caught out by Serena for his hypocrisy, Dan and Serena retreat into the house to talk. They decide to meet up later at the beach, where they spend the night together.
The following morning, Serena and Dan decide to halt any discussions on their relationships until they are back in the city. After some confusion between who was to take the train and who was to take the bus, they both ended travelling back to the city on the bus. This creates tension between the two, and as Serena gets up to use the bus bathroom, she trips and falls on Dan. She pulls up Dan and the two go into the bathroom to "fool around". The two decide to keep their rekindled relationship a secret until they were sure of themselves. Back in the city at Blair's "Welcome Back" party, Nate discovers the two in an embrace, revealing publicly that their relationship is "back on".
In "The Dark Night" during the city-wide blackout, Dan and Serena talk about their problems while they were both trapped in an elevator. After a long talk (and a bit arguing), they realized they keep having the same fight, and break up.
During their return to school, Serena and Dan are now friends until he meets a transfer student named Amanda Lasher. They share the same interests and have perfect chemistry. Blair attempts to keep them separate by making her one of her entourage so that Serena wouldn't have to suffer seeing them together and so she also wouldn't have to lose the competition of rebounding. Serena meets Amanda and nearly makes a fool out of herself. Blair forces Amanda to join them for lunch. Dan, thinking that Serena is trying to keep him away from Amanda gets Serena, who was reasoning with Dan why it happened, irritated when he kept implying that she was the one who forced her to lunch. Dan and Amanda's relationship goes public much to Serena's irritation. Serena then invites Dan and Amanda to go out later that night which turns into as disaster as Dan and Amanda continue to make her feel out of place. Isabel and Penelope, after witnessing Dan go out with Amanda that led to the relationship getting public, offers Serena several suitors, one in particular is a Dalton lacrosse team captain. Serena's new suitor fails to impress Dan. Dan thinks that Serena used the lacrosse team to rub it into his face. Later on, Penelope ruins Amanda's hair with a little help from Chuck. Dan runs to Amanda's aid who brushes him off saying that she no longer wants to see him. Dan gets into an argument with Serena and leaves. The next day, he is shunned because of Serena's rise to Queen Bee status.
Dan's recommendation from Noah Shapiro, a writer, leads him to find new inspiration in the form of Chuck Bass. Noah suggests he use Chuck to fuel a new story. Dan does so and loses his shoes in the process. He ends up creating the character of 'Charlie Trout', a character that impresses Noah and asks him to find out what makes him tick. That night, Chuck mistakes a woman of being a prostitute and nearly gets punched until Dan defends him. The two end up in jail and Chuck confesses that he thinks his father hates him because he thinks he killed his mother at Chuck's birth. Chuck gets out a jail with some help from his lawyers and discovers Dan's reason as to why he was hanging out with him. Chuck gets angry at Dan and leaves him, saying that he lied about his mother's death and that she died in the Andes in a plane accident. Dan calls Noah Shapiro who bails him out. Dan confesses that he thinks it's wrong to exploit people for art and that he'd rather be safe. The two part ways and in bad terms.
During a visit to Yale, Dan's recommendation for Yale admission goes awry with his recommendations as his only acceptance came from J.L. Hall. Jeremiah Harris and Noah Shapiro likely declined to write for Dan's recommendation. The Dean of Admissions suggests that he find someone to read his work. Dan then finds out Nate used his name to get into a fling with Jordan, a co-ed girl who eventually helps Dan get his work to other literature professors. Chuck's revenge on Dan then turns the tables on Chuck who loses Nate as a friend after Nate finds out about what he did to Dan.
Dan and Nate begin a friendship much to Chuck's chagrin. During an invite to play soccer, Dan tries to visit Nate at his house and finds out about Nate's financial situation. Nate has been squatting in his own house that got seized amongst other property of the Archibalds. Dan invites Nate to a family dinner and tells Nate that he knows about Nate squatting in his own house. Nate leaves angry. Rufus talks to Dan and tells him that Nate needs help even if he doesn't want it, seeing as Nate has no one to turn to for help. Dan finds Serena outside Nate's house and repair their relationship. The two become friends. Dan convinces Nate, who moves in after his house got seized.
With Blair's situation with Chuck now out of hand, Serena gets Dan to help Blair and tells her that wearing him down would help her situation. Later on, he continues to help Blair by telling her that she should intercept him at home. With Blair desperate, he tells Blair that she should take a risk and say those '3 words, 8 letters' anyway. During Aaron's opening at Rufus' gallery, Vanessa confesses that Blair and Chuck used her in one of their sick games. Dan ruins Blair's opportunity to tell Chuck that she loves him and tells Serena what happened. He said that the two were bound to break up eventually and that what they did to Vanessa was sick. Serena says that Vanessa's matter is different and that Chuck and Blair do love each other. He later apologizes to Serena and get into an honest conversation. Serena confesses her feelings for Aaron and Dan gives her his blessing to date other people.
Aaron and Serena start to date each other while Dan still realizes that he till has feelings for Serena. Aaron has been concerned that Serena leans emotionally more on Dan than him. He asks her to come with him to Buenos Aires for a holiday. Serena asks Dan for his opinion, and he tells her to go. At Eleanor's wedding however, Dan confronts Serena about his feelings for her. Because she knows about her mother and Rufus' relationship, and because she has already said yes to Aaron, Serena turns Dan down and goes ahead with Aaron's trip. Serena breaks up with Aaron during the holiday and Dan and Serena meet with each other and they both decide there are better off back together. Dan soon learns he shares a sibling brother with Serena and becomes ridden with guilt when Rufus asks him to let Lily tell Serena the news.
Serena and Dan attempt to return to normal, but find this difficult with Lily and Rufus, who have decided to go public with their relationship. Dan, and Serena both get acceptance letters to Yale but Serena decides that Yale isn't for her and applies to Brown and gives her spot to Blair who was wait-listed. A new teacher named Ms. Carr teaches at Constance Billard and there is an obvious attraction between them. Blair is so willing to get back at Ms. Carr for giving for a B on a paper that she sends in a rumor to Gossip girl concerning a secret relationship going on between Dan and Ms. Carr. Serena herself is becoming suspicious when she sees Dan and Ms. Carr discussing Dan's paper together but believes Dan when he says nothing is going on between the two of them. Serena then sees Dan and Ms. Carr in a coffee shop together. She cannot hear what they are saying but in fact Dan is only comforting Ms. Carr who is in tears about the slanderous teacher-student relationship rumor. Serena captures a photo of Dan touching Ms. Carr's face in an intimate way and shares the photo with Blair although she has doubts about the authenticity of the situation.
Blair enters a parent-teacher meeting at Constance Billard and presents the incriminating photo at hand. Dan explains to Serena nothing happened between him and Ms. Carr but it is too late. Ms. Carr is fired. Serena and Dan's relationship hits another dead end when things become too much for them to take. First their parents start dating, then they find out they shared a half brother together, and now the mess of the rumor.
Serena goes to apologize to Ms. Carr while admitting regretfully that she was the one who gave the photo to Blair. Later Dan visits Ms. Carr and starts to apologize for how things went down but then she kisses him. Dan, at first, is surprised at her actions. Ms. Carr points out that she doesn't teach at Constance anymore so there is no student-teacher relationship. So Dan obliges and they have sex in the dark of the room.