Demi first arrived in Walford at the age of 13, and immediately became the talk of the Square due to her pregnancy. Feisty and troublesome, she loved to cause trouble with her twin brother Darren. The duo regularly truanted from school, and at one time even broke into the Slater house, scaring Garry.
Nearly two months after her arrival, Demi collapsed in agony in the playground after her waters broke. Scared and alone, she begged Pauline Fowler to help her. Despite not believing Demi, after Demi scammed her days before, Pauline eventually realised the teenager was serious. With Pauline's help, she gave birth to Aleesha Miller on 29 October 2004. Later that evening, Aleesha's father and Demi's boyfriend Leo Taylor arrived with his father Ray. Clearly influenced by his vicious father, Leo was told he was to have no further contact with Demi or Aleesha.
Demi found it hard to bond with her baby daughter due to her mother, Rosie interrupting every time the mother and daughter interacted. Rosie insisted that Aleesha slept in her and Keith's room, as well as taking on full childcare. But on Christmas Eve, Demi told her that she wanted to be a proper mother to her daughter. Rosie apologised for taking responsibility for her granddaughter. Demi matured into a caring, responsible mum after this.
When it was revealed that Keith Miller was illiterate, Demi found it difficult to trust him alone with Aleesha. She tried to drop out of school in April 2005 to take on full responsibility for Aleesha.
In May 2005, the Millers returned to their former home in a block of flats whilst a rat problem in the square was being dealt with. Demi was reluctant, as it meant she was likely to bump into Leo. Upon their arrival, Leo's father Ray and his wife Trisha were the first people they saw, and they received a cold reception. Whilst taking Aleesha for a walk, Demi was teased by some of Darren's old friends, who mocked her and referred to her as "Vicky Pollard". Upon heading back to the flats, Demi saw Leo and tried to escape in a lift, but Leo stopped her. They realised their split had been due to their parents meddling and denying contact between the two, and they reunited. Demi told Leo that it was Aleesha's christening the following day. Despite appearing reluctant, Leo and Trisha did indeed attend the christening, however the service was interrupted by Ray, who viciously dragged his wife and son away whilst a heartbroken Demi looked on. The christening continued back at Clint's flat, and Demi and Leo met later that evening from their balconies. As the feud between the Millers and the Taylors escalated, Leo planned to run away, and begged Demi to come with him. Demi agreed, but Leo abandoned her when she insisted on taking Aleesha. Demi returned to Walford, devastated, but was delighted when she met Leo near the swings. Leo promised her he'd keep in touch, and would wait as long as it took to be together.
After numerous meetings in secret, Darren overheard Rosie and Keith saying that Leo's parents were planning to take him to live in Scotland. Darren told Demi and Leo, who at first refused to believe him, but soon realised that he was telling the truth. Demi and Leo spent one final day together, and vowed never to love anybody else. But the following day, Leo turned up on the doorstep telling Demi that he'd run away, and wanted Demi and Aleesha to go with him. After saying goodbye to Darren, Demi took Aleesha and ran away with Leo.
For days, the Millers desperately searched for the missing trio. After a phone call home in which she overheard the Millers and the Taylors arguing, Demi thought that she couldn't go home because they hated her. Leo promised to take Demi and their baby to Southend for a new life together. The young family were stuck in a squat struggling to afford to survive. Demi wondered about how long they could stay there for, as the water was far too cold to bath Aleesha in and neither of them were earning money. Leo promised to get a job as a builder, but Demi mocked the plan as he was underage. After Leo dropped Aleesha's last remaining milk powder all over the filthy floor, Demi realised that the only way to secure her daughter's safety was to send her back home. She phoned Pauline, and told her to meet her at the church Aleesha was christened at, where she handed over her daughter in a tearful farewell. Pauline brought Aleesha back to the Millers, leaving Rosie even more concerned about her daughter's welfare. Leo begged Demi to let him sell drugs one final time to make some money. Reluctantly, Demi agreed, and waited for Leo to return. Gavin, the man who gave Leo drugs, tried to kiss Demi, and told her the only way to survive was if she turned to prostitution. Shocked and upset, Demi turned to heroin. Demi and Leo take drugs.Elsewhere, Leo was beaten up for his drugs, and left in a heap on the floor, feeling hopeless. Back in Walford, Trisha defied her husband and united with the Millers to find their missing children. Mickey and Darren narrowly missed Leo, who found one of their posters appealing for their return. Realising they were loved after all, Leo raced back to the squat to tell Demi, and found her unconscious with the heroin by her side. Believing she'd killed herself, he overdosed on the remaining heroin just as Demi woke up. Leo collapsed in her arms just as Keith, Rosie and Trisha found them.
Demi and Leo were rushed to hospital, where the Miller/Taylor feud threatened to erupt again. But after Ray promised to beat Leo for this incident, Trisha had had enough. She told Ray that as soon as Leo recovered, she was taking him to her sisters away from his violence and they were finished. At that moment, a doctor turned up and told them that Leo had died of his overdose. As Rosie entered the room, Demi guessed that Leo was dead, and turned away to cry.
After the traumatic death of Leo in August 2005, Demi was happy to be reunited with her family. It took a while to forgive them for hating Leo, but soon she was back hanging out with her twin again. After a few weeks of bullying, she started to settle back into school again. In October 2005, Demi celebrated Aleesha's first birthday and she was also joined by her older half sister, Dawn Swann.
Christmas arrived, and Rosie's ex-husband Mike< turned up to visit Dawn and Mickey. He also told Rosie that his mum, Nora was very ill. Nora later died, bringing the two closer together, and after tiring of Keith and the family, Rosie kissed her ex-husband, but immediately regretted it. She later planned to take Demi and Darren away from Keith and live a new life together, but instead backed out and told Mike to leave them alone. Not long after, Keith found out that Rosie had kissed her ex-husband and planned to run off with him, and left her. Demi bunked off school on her 15th birthday and visited her dad, leading to Keith returning to be with the family for a brief period but he and Rosie split up again.
Keith moved back in when he proposed to Rosie. The wedding ceremony took place on 4 July 2006 at Walford Registry Office. Rosie couldn't go through with the wedding and jilted Keith during the ceremony. Rosie planned to move away with Darren, Demi and Aleesha to take on a new job as housekeeper and start a new life without Keith. Although Darren was less than happy at the news, Demi wanted to bring Aleesha up somewhere different from Albert Square. Rosie, Demi and Aleesha left Walford on 7 July 2006.
Since her exit from Walford, Demi has been visited by the Miller family twice - once by her brother Darren, who stayed for two months from June to August 2007, and once by her sister Dawn, who after giving birth to her daughter Summer< travelled to the Cotswolds in order to introduce her baby to Demi and their mother Rosie in July 2007 for a month. After Dawn returned to Walford in August, she revealed that Demi had passed all of her GCSE's but had since left school in order to be a full-time mother to her daughter Aleesha. When Darren returned from the Cotswolds a few weeks later he also revealed that Demi had got herself and Aleesha a boxer puppy that they named Clara.
In July 2008, Demi's brother Mickey and father Keith joined her, Aleesha and Rosie in the Cotswolds when Mickey took a job as a concierge at the hotel where Rosie worked as a cleaner.
In August 2008 it was revealed that Demi would be unable to attend her sister Dawn's wedding to Jase Dyer as she was suffering from tonsillitis. However after Jase's murder, Dawn and her daughter Summer returned to the Cotswolds to stay with the Miller family for a month in order to grieve and move on with her life.
Demi was mentioned again in January 2009, in a conversation between Darren and Libby about Demi and Darren's 18th Birthday.