Chuck grew up with Upper East Side elites Nate Archibald, Blair Waldorf, and Serena van der Woodsen. His father is Bart Bass, an entrepreneur, a successful businessman, who made a name for himself, rather than being born into money like Chuck’s friends’ parents. Chuck is often described as the bad boy of his circle, and although he can be manipulative, we learn that he has a sensitive and caring side to him, especially for those he cares about like his best friend Nate and love interest - Nate's ex-girlfriend - Blair Waldorf, who is the first girl he admits to loving.
Chuck is notorious for being a playboy and is frequently shown sleeping with hired women for recreation. He also attempts to hook up with Serena and Jenny Humphrey in the pilot episode. Chuck was the only one who knew about Serena and Nate’s affair, but didn't tell anyone until after Serena came back, at the brunch his father was hosting.
Chuck is at first shown as being fiercely loyal to his best friend, Nate Archibald. When Nate leaves Chuck's party angry and frustrated, Chuck winds up rescuing him from Carter Baizen and a risky poker game. He also said in a season one episode that the only things that matter to him are money, the pleasures money can buy him, and Nate.
However, Chuck unintentionally betrays Nate by sleeping with Nate's long-time girlfriend, Blair Waldorf, a few hours after they broke up. Blair had come to Chuck for solace after the break-up, the two of them being close friends and oftentimes, partners in crime. Upon seeing Blair boldly perform a burlesque dance at Chuck's club Victrola immediately after breaking up with Nate, he becomes immediately smitten with her, and the two have sex in his limo on the ride home. It is clear that Chuck has a respect for Blair that he lacks when it comes to other women, thinking of her not only as his best friend's ex, but as a close friend herself. This is evident when before he deflowers her, he asks, "You sure?" to which she answers him with a kiss.
However, the morning after, Chuck finds her confessing at a Catholic church, and she tells him that they should never talk of what transpired between them, despite Chuck clearly not agreeing to this. At Blair's birthday party Chuck admits his feelings for her; that he cannot sleep for thinking about her and the presence of butterflies "fluttering" in his stomach. Blair isn't pleased with his sudden attention or amorous feelings, and just wants him to leave her alone. After Nate fails to reconcile with Blair on her birthday as she had hoped, Chuck consoles her and wins her over by giving her the $30,000 Erickson Beamon necklace she had put on hold that morning, expecting Nate to give it to her as her birthday present. They wind up having sex again, but unbeknownst to them, Serena spots them together. Serena later confronts Blair about this in "Blair Waldorf Must Pie", but Blair gets defensive and claims that her past two encounters with Chuck were simply "revenge sex".
Chuck and Blair carry on a secret relationship for two weeks, while Nate begins to reconsider his decision to break up with Blair. In Hi, Society, Nate expresses his newfound interest in Blair, who he feels has changed. Later, in the middle of a make-out session between Chuck and Blair in her room, Nate comes to visit her and asks her if the two of them can go to the debutante ball as friends. At first, Blair is reluctant, but caves when Nate shows her the heart pin she sewed on his sleeve before. As they prepare for the ball, Nate senses that Blair is not really interested in him and may be seeing someone else, so he asks Chuck to find out who the secret guy is. Meanwhile, Blair has decided to end things with Chuck in light of her reconciliation with Nate, but he clearly has strong feelings for her. Hurt and rejected, Chuck decides to kill two birds with one stone and snaps a photo of Carter Baizen leaving Blair's apartment after picking up his coat. He convinces Nate that Carter Baizen is Blair's new guy, and an enraged Nate attacks Carter at the ball getting himself kicked out in the process, just as Chuck had planned. However, Blair cottons onto Chuck's machinations and promptly finishes things before going to look for Nate. Chuck looks for Blair in a last-ditch effort to explain himself, and ends up seeing Blair going to bed with Nate. A heartbroken Chuck leaves New York to avoid the pain of seeing Blair and Nate together.
Nate and Chuck spend Christmas in Monaco, where Chuck blackmails Blair with their secret in order to keep her away from Nate. However, she eventually realizes that revealing their tryst would hurt him too, so she informs him that it's over. Seething with jealousy, Chuck retaliates by writing an anonymous text to Gossip Girl detailing that Blair has slept with two guys in one week. The news spreads quickly, which ruins Blair and comes back to haunt him. After seeing the Gossip Girl news about Blair, Nate asks Jenny if it is true, and Jenny, who overheard Serena telling Dan, tells him that Chuck slept with Blair. As a result, Nate and Chuck have an explosive fight outside the school and Nate ends their friendship. At the end of the day, a devastated Blair returns to Chuck, saying she has no one else. To her dismay, he rejects her rather cruelly and tries to remain loyal to an estranged Nate.
In the meantime, Chuck's father and Serena's mother have become engaged, causing them to become step siblings. His father ended up kicking him out of the apartment to live back in his suite after Bart and Lilly became engaged and moved in together. Chuck constantly annoys Serena, but takes a brotherly liking to Eric, Serena's little brother, looking out for him and talking to him about his sexuality.
When Serena's old friend Georgina returns, Chuck does his best to help his soon-to-be stepsister through the difficult time. He eventually teams up with ex-friend Nate and ex-lover Blair to save Serena and counter Georgina's evil. With help from Blair, Nate, and Chuck, Georgina is eventually turned in to her parents and sent to a bootcamp. Serena, Nate, Blair and Chuck are dubbed "The Non-Judging Breakfast Club."
At his father's wedding to Serena's mother, Chuck alerts Nate when he sees the Captain, Nate's father, appearing to buy cocaine. This causes the two friends to make up and, upon realizing Chuck is actually in love with Blair, Nate gives his blessing. While giving his best man speech, Chuck sees Blair and puts down his notecards, instead talking about true love and perseverance, inspired by his complex relationship with her. While dancing with Blair, he apologizes for the terrible things he said to her in the past and tells her that she doesn't belong with Nate. She replies, "You don't belong with anyone" and they kiss. He vows that this time they will take it slow and do it right, showing a softer, more romantic side of him. The two seem to be going strong until the day that he and Blair are set to leave for Tuscany for summer vacation when his father talks to him about becoming a better man and changing because of his relationship with Blair. Now having second thoughts about spending the summer with Blair, he ditches her for their trip to Tuscany, saying he cannot make the flight, and offers a rose to Lily Van Der Woodsen's interior designer, Amelia. Blair, unaware of Chuck's intentions, believes that his father must be running late and Chuck will probably catch the next commercial flight.