Liam was four years younger than his sister. Shortly after his birth, their mother had an affair with their father's best friend. When their father learned of this he left Perivale, taking the infant Liam with him. Dorothy was only four years old at the time, so Audrey simply never told her about any of it and raised her as an only child.
In 1993, Liam's father had a heart attack, and finally told him he had a sister. Liam tracked down Audrey but she turned him away, telling him only that his sister had begun calling herself "Ace" and had disappeared years earlier, and giving him a photograph of her. Before Liam returned home, his father had died.
Liam finally met Ace in Ibiza on 14 May 1997, where he had gone with his friend Caitriona Henderson. He recognised Ace as his sister Dorothy from Audrey's photograph. Eventually, Liam revealed who he was to Ace and told her everything, proving his claim by showing her a letter from their father. Ace, overwhelmed by how different her life could have been if not for her mother's lies, told Liam everything about herself and her travels with the Seventh Doctor in the TARDIS.
Ace soon decided that she wanted nothing to do with Liam. Having worked hard to sort her life out, she didn't feel there was any room for a brother in it. However, after Liam and Caitriona helped the Doctor and Ace foil the plans of "angels" Jude and Gabriel, Liam and Ace put aside their differences and spent several days catching up. When it came time to part ways, Liam elected to stay on Earth with Caitriona, sure he could never handle the kind of life Ace lived. Ace continued on her adventures with the Doctor, with the knowledge she now had something to look forward to returning to one day. (AUDIO: The Rapture)
Liam described himself as "Christian sci-fi Muppet" to his sister based on the fact that he was a regular churchgoer as well as a science fiction fan. Specifically, he was a fan of The X-Files and Professor X. (AUDIO: The Rapture)