Adam Smith is the titular main antagonist in the Torchwood episode "Adam". He was a mysterious entity that could travel into people's dreams and memories and even change them, mainly altering their memories to include him, as he could exist as long as people remembered him.
He was portrayed by Bryan Dick.
Adam decided to use his powers on the Torchwood group since he thought their memories were unique and interesting, he later gained the trust of Jack Harkness with memories of his presumed deceased brother Gray. He changed Owen Harper into being a more vocal and emotional man and even made him slightly similar to Tosh.
He made Toshiko Sato have deep feelings for him and made her think they had a relationship for a long period of time, using this power to manipulate her into having sex with him. He also made Gwen Cooper see him as a good friend but accidentally made her forget about her husband Rhys. Finally, after Ianto Jones realised that there was no mention of Adam in his diary, Adam implanted a false memory to make Ianto believe he was a serial killer.
Jack then erased footage of him from the security recordings and in an attempt to stop Jack from killing him, he implanted himself into another memory of Jack and his brother when the two were young. Jack took an amnesia pill and walked away, leaving Adam disheveled and tearful before he went into a seizure like state and crumbled into nothing, disappearing forever with nobody from the team remembering him.