Rosie's mother committed suicide when she was young, leaving her with her brother, Clint, and other siblings. Rosie was married to Mike Swann, and they had two children, Mickey and Dawn. While they were still very young, Mike left Rosie and wasn't seen for many years. Rosie later met Keith Miller, who took on her children as his own before having twins, Darren and Demi. He cannot read and is work-shy. He tends to sit around watching television all day, making excuses to Rosie as to why he does not go and find employment.
The Miller family move to Walford in 2004. In November 2005, Darren finds thousands of pounds belonging to Jake Moon, which Alfie Moon has accidentally thrown out in the rubbish. Darren uses the money to buy a mobile phone, and spends it mostly on his girlfriend, Kylie, who eventually splits up with Darren because he is unable to buy alcohol. Darren shares the money with his twin sister Demi, but it is found by their older half sister Dawn's father, Mike Swann. Mike steals the money, leaving the Millers with nothing.
Darren finds employment at Kevin Wicks's car lot, and decides to stay in Walford when his parents split up in 2006, and his mother and sister move to the Cotswolds. He becomes friendly with Libby Fox and they perform numerous pranks on their families together; an attraction develops, and they begin a relationship. Darren gets involved in a car scam when Kevin starts selling stolen cars that turn out to be dangerous cut and shuts. When Kevin dies due to an accident in one of the cars, Darren takes over control of the car lot. Darren blames Phil Mitchell for Kevin's death, as he forced Kevin to sell the cars. Darren subsequently gets involved in a feud between Phil and Jack Branning (Scott Maslen), helping Jack to trick Phil into a confession about the stolen cars. Darren and Libby's relationship continues into 2008. Libby's mother, Denise Wicks, feels that Darren is a bad influence on her daughter, and when Libby opts not to apply to Oxford University so she can stay with Darren, Denise persuades him to end the relationship. Darren dumps Libby to satisfy her needs, leaving her heartbroken. She eventually discovers the truth; she reunites with Darren. Darren makes Libby promise that she will not ruin her future and persuades her to apply to university.
Keith and Mickey Miller leave Walford to join Rosie and Demi in the Cotswolds. Darren's last relatives in Walford, Dawn and her daughter Summer, also decide to leave with Garry Hobbs. Libby then goes to university, but returns when her father Owen Turner is released from prison. Darren asks for Owen's permission to propose to Libby, and she accepts his proposal. They celebrate in The Queen Victoria pub, where Denise overhears them and tells them they cannot be engaged. However, Darren soon finds out that he is the father of Heather Trott's baby, George. Darren tells her that he will support George with money but nothing more. However, Darren begins to adapt to the idea of being a father to George. Darren gives Heather money he needed for a holiday with Libby. Heather returns it, in a card which Libby opens. Libby later overhears Heather and Shirley Carter talking about Darren. Devastated, Libby tells Darren in front of everyone that she knows he is George's father. Libby then goes back to Oxford. When she returns to Walford, Darren is upset to learn that Libby is dating Adam Best. Darren meets Jodie Gold, and the two start a relationship. Darren and his flatmate Minty plan to attend the 2010 FIFA World Cup final. Minty gives the tickets to Darren to look after, but they end up destroyed in the wash, when Darren leaves them in his pocket. An enraged Minty orders Darren to move out of the flat. When Abi Branning hears that Darren is homeless, she convinces her father Max to let him stay with them. Abi starts to fancy Darren, and attempts to sabotage his dates with Jodie. On the first night that Darren attempts to get intimate with Jodie, she screams at the sight of his naked body, and runs away. Darren confronts her, and she apologises, but says that because Darren is not Jewish, and therefore not circumcised, she would find sex strange with him. Initially reluctant, Darren makes a call to book a circumcision. He does not tell Jodie he is having the operation and tells her he is going on holiday while he recovers. After the procedure, Jodie's father Harry mistakes him for Max and knees him in the groin, due to Max's affair with his wife Vanessa. The next day, Jodie sees Darren and tells him she no longer wants to see him because he lied. Darren tries to win Jodie back by making her laugh and tries to explain that he had a circumcision but she will not listen to him because he did not tell her about Max and Vanessa.
However, she finally reads a letter he wrote and they reunite. They go on to have sex for the first time. Harry later returns to Walford with Vanessa to get to know Darren better. Darren worries when Max jokes that Abi must fancy him, not realising it is true. Max and his other daughter Lauren later tell Darren it was a joke and she would never fancy him. Abi however remains keen to show affection to Darren. She uses the opportunity of the death of her cousin Billie Jackson to get physically close to Darren to seek emotional support. Although Darren is at first willing to offer this support he later appears uncomfortable with Abi's close attention. When Harry asks Jodie to move out, she moves in with Darren. Harry then humiliates Vanessa and Darren confronts him but learns that Harry is not Jodie's real father. Darren begs Harry not to tell Jodie is it would break her heart. He then worries about what to do as he does not want to lie to Jodie. Later when Jodie says Harry has phoned her, Darren assumed he has told her when he has not. Darren then inadvertently reveals the truth to Jodie.
During a New Year's Eve party, a drunken Jodie persuades Darren to tell her he wants to marry her. Darren is unsure at first but goes to propose, but she stops him as she was not serious. Max decides to relaunch the car lot with Jack as his new business partner. Jealous, Darren tries to sabotage the relaunch by not sending out the flyers but Max finds out and sacks him. Darren blames his sacking on Jodie, so she dumps him. At the R&R nightclub, Darren kisses a girl, causing Jodie to storm out. The next day he tries to apologise, saying it meant nothing, but Jodie will not listen. Abi finally confesses her feelings to Darren and tries to kiss him, but he rejects her because of her age and because he still loves Jodie. Jodie overhears him telling Abi how much he loves Jodie, and then he tells Jodie that he would change nothing about her and proposes. Darren works for his friend Tamwar Masood as a waiter until Vanessa convinces Max to reemploy Darren, which she does for Jodie's sake. When Heather suffers carbon monoxide poisoning, Darren makes a responsible decision to take custody of George for his health and safety. Whilst in Jodie's care, George has an accident at the salon and Heather wants to take care of him again but Darren will not let her and tells Heather that he is going to court to gain permanent custody of him. After the accident, Jodie decides that she is too young to be a stepmum, and, rather than make Darren choose between her and George, ends their relationship.
Darren realises that being a single parent is not easy, and drops the custody case. He and Jodie get back together. He later plans a trip to see his family, not wanting Jodie to go, but Jodie does not want to go anyway. When he returns, he starts stealing money from work in order to pay for the wedding. Jack discovers this and informs Jodie, who says the wedding is over until she speaks to Darren. However, Darren then gets drunk with Jodie's friend and bridesmaid Lauren and they kiss and have sex at the car lot. Jodie later arrives at the car lot and sees Darren and Lauren together but does not say anything and they are unaware that it is her that saw them together. Jodie later admits to Darren that she saw him and Lauren together but has forgiven him. On the wedding day, Vanessa finds out from Syed what has happened between Darren and Lauren and goes to confront Darren telling him he is not good enough for Jodie, comparing him to Max and Harry. Darren is on his way to the wedding when he changes his mind and goes to tell Jodie that he cannot marry her. After saying goodbye to his friends, Darren visits Jodie and tells her that he still loves her but is not worthy enough to marry her and then leaves Walford in the wedding car that Mickey had hired for them. As he is driving away, a heartbroken Jodie runs after him and catches the congratulations balloon which had fallen off the car.
In November 2011, Tamwar reveals that Darren is staying with Libby in Oxford. Jodie realises she and Darren were meant to be together so leaves Walford to find him. In March 2012, Social Services try to find Darren after Heather is killed and George is left without his parents. They are able to trace him to India but are unable to find any more information so George is sent to live with his godmother, Shirley Carter. Phil eventually is able to contact Darren and tells him to come back to England. Social Services then collect George from Shirley and Phil and take him to Darren.