Demi arrives in Walford at the age of 13 and immediately becomes the focus of local gossip due to her pregnancy. She is feisty and likes to cause trouble with her twin brother Darren (Charlie G. Hawkins). Nearly two months after her arrival, Demi collapses in agony in the playground after her water breaks. Scared and alone, she begs Pauline Fowler (Wendy Richard) to help her. With Pauline's help, Demi gives birth to Aleesha Miller (Freya and Phoebe Coltman-West) on 29 October 2004. Later that evening, Aleesha's father and Demi's boyfriend Leo Taylor (Philip Dowling) arrives with his father Ray (Dorian Lough). Ray is extremely hostile, and Leo is told he is to have no further contact with Demi or Aleesha.
In May 2005, the Millers return to their former home while a rat problem in the square is being dealt with. Demi is reluctant, as it means she is likely to see Leo. Soon after their arrival, Demi takes Aleesha for a walk and sees Leo. They realise their split has been due to their parents meddling and denying contact between the two, and they reunite. Demi tells Leo that it is Aleesha's christening the following day. Despite appearing reluctant, Leo and his mother Trisha (Cathy Murphy) attend the christening, however, the service is interrupted by Ray, who viciously drags his wife and son away. Demi and Leo meet later that evening from their balconies. As the feud between the Millers and the Taylors escalate, Leo plans to run away, and begs Demi to come with him. Demi agrees, but Leo abandons her when she insists on taking Aleesha. Demi returns to Walford but is delighted when she meets Leo near the swings. Leo promises her he will secretly continue to see her.
When Leo's parents plan to take him to live in Scotland, Demi and Leo take Aleesha and run away. The young family then live in a squat, struggling to afford to survive. Demi realises that the only way to secure her daughter's safety is to send her back home. She phones Pauline and tells her to meet her at the church Aleesha was christened at, where she hands over her daughter in a tearful farewell. Leo begs Demi to let him sell drugs to make some money. Reluctantly, Demi agrees, and soon turns to heroin herself to numb the pain. Leo finds a poster of him and Demi, and realises their families are looking for them. Leo races back to the squat to tell Demi and finds her unconscious with the heroin by her side. Believing she'd killed herself; he overdoses on the remaining heroin just as Demi wakes up. Leo collapses in her arms just as Trisha and Demi's parents, Keith Miller (David Spinx) and Rosie Miller (Gerry Cowper) find them. Leo dies later in hospital in August 2005. Demi is reunited with her family and goes back into school. In October 2005, she celebrates Aleesha's first birthday, and she is also joined by her older half-sister, Dawn Swann (Kara Tointon).
Rosie plans to move away to the Cotswolds with Darren, Demi and Aleesha to take on a new job as a housekeeper and start a new life without Keith. Darren decides to stay in Walford, but Demi wants to raise Aleesha somewhere different from Albert Square. She leaves Walford with Rosie and Aleesha on 7 July 2006.
On 9 December 2024, Mickey returned briefly to Albert Square, revealing that Rosie and Dawn are currently working at their new family business, ‘Miller’s Villas’ in the Cotswolds. It is hinted that Demi and Aleesha also help out.
