He was conceived after Kat was raped by her uncle Harry Slater. As Kat was so young, she was told she only had one baby, a daughter who was born first. Minutes later, another baby was born (a boy) unexpectedly, but Kat was told it was the afterbirth. Kat's mother Viv knew that Kat had been raped by Kat's father Charlie's brother but kept quiet. Together, Viv and Kat's Nan Mo named the baby Luke Slater, but decided to give him away, as Viv thought that Luke might have turned out like his father, and she wanted to keep Zoe as she was born first and thought it would be right, also Viv said she could not have passed off being pregnant with twins. Luke was later adopted by Iris and Peter Dolan and was named Dermott.
Dermott has never appeared in EastEnders, only Redwater.