Silent Witness

Sam Ryan () – series 1–8, 25. Sam originally lived and worked in Cambridge, but moved to London at the end of series three after she was offered the job of professor at a university. Sam departed and returned home to Northern Ireland in the second episode of series eight, "A Time To Heal", in which a member of her family became a suspect in a murder case.

Leo Dalton () – series 6–16. Leo first appeared in the episode "The Fall Out", where he was a doctor. However, he was promoted to professor after former professor Sam Ryan left. In the episode "Ghosts", both his wife, Theresa, and his daughter, Cassie, are killed in a car accident. He began a relationship with fellow professor Janet Mander in "Death's Door", but ended their relationship in the episode "Redhill". Leo often took a strong opinion on cases and became emotionally involved. He died in the series 16 finale "Greater Love", when he sacrificed himself to save many others from a terrorist bomb explosion. He briefly appeared in a flashback in the last episode of series 20.

Harry Cunningham () – series 6–15. Harry started out life as a junior doctor, who worked as an apprentice to Sam and Leo. However, he soon qualified as a pathologist, and has worked on equal footing with the team for a number of years. Harry was single, and lived alone, but has had several romantic relationships, including an ongoing 'will they-won't they' relationship with his colleague Nikki. Harry left the team to accept a professorship in New York at the end of series 15. As this was decided after filming was completed, and the stories were reordered, with "And Then I Fell in Love" airing last, where it was originally scheduled as the second episode, his departure was never on screen.

Nikki Alexander () – since series 8. Originally appearing in the episode "Nowhere Fast", Nikki was originally assigned to the Lyell Centre to defuse the tension between Harry and Leo after Sam's departure. However, Nikki soon became a permanent fixture within the team, and although she features more prominently in the series, holds a lower rank in pathology than Leo. Despite her native home being South Africa, Nikki regards the United Kingdom as her second home, as the reason for her departure from the country lies solely in the hands of her father, Victor, as explained in the episode "Double Dare". Nikki often flirted with colleague Harry, and even invited him to stay at her house after his flat blew up in an explosion. Nikki and Harry were in an ongoing 'will they-won't they' relationship until he left in series 15. As well as this, she developed a close relationship with Leo and looked up to him as a father figure. As Leo and Harry gradually left, Nikki has since become the series central character.

Jack Hodgson () – since series 16. Before his arrival at the Lyell Centre, Jack was a forensic scientist for the police, working on regular murder investigations. However, when he is called out to the scene of a man's suspicious death in "Change", both Nikki and Leo spot his potential, and looking for a senior colleague to replace Harry, decide to offer Jack a job as the centre's forensic expert. Jack is close friends with colleague Clarissa Mullery, whom he invites to work with him at the Lyell Centre, much to Leo's surprise. In his spare time, Jack is a cage fighter, and splits his home life between forensic research and training for his next fight.

Clarissa Mullery () – series 16–23.^[12] Clarissa is Jack's personal lab assistant, who first appears in the episode "Change", when Jack invites her to work at the Lyell Centre with him, much to Leo's surprise. She is disabled by an unspecified condition and uses an electric wheelchair. She has a very cheeky side, making a quip at Leo after he fails to recognise her when she first arrives. Clarissa had clearly worked for Jack for a long period of time before his appointment at the Lyell Centre, but the exact period of time is unknown.

Thomas Chamberlain () – series 17–23. Thomas Chamberlain took over as head of the Lyell Centre from Leo who died at the end of series 16. He was an experienced forensic pathologist with a renowned reputation in toxicology and is described as charming, charismatic and socially shrewd. His first encounters with Jack, Nikki and Clarissa did not go well, but they gradually warmed to him. It is revealed in the last episode of his first series that his wife left him just before he started working at the Lyell Centre, taking their daughter with her. He died in the series 23 finale "The Greater Good" whilst investigating a nerve agent in the pathology suite of the Lyell Centre.

Simone Tyler () – series 24–25. Simone is a meticulous forensic ecologist, who makes the jump from the museum to the mortuary at the Lyell Centre. Simone leaves at the end of season 25 to look after her nieces in Trinidad after learning about her estranged sister's death.

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