Max Tresilian was a human who joined Pilgrim and befriended Suzie Costello. He served as an unknowing Trojan Horse to be used as part of her plan to bring herself back to life if she needed to kill herself.
Once a week for two years, Suzie would discuss her feeling about her job at Torchwood Three with him, and then wipe his memory with retcon. Ingesting these large amounts of Retcon eventually drove Max insane, and she re-modelled his mind around certain poems by her favourite poet, Emily Dickinson. After Suzie shot herself and had not met him in three months, Max killed three fellow "Pilgrims", and was captured in the Wolf Bar by Torchwood. He was then taken to the Vaults, where Torchwood discovered their name drove him mad for exactly 10 seconds, before returning to silence instantly.
As Suzie made her escape with unwitting accomplice Gwen Cooper, Max began manically repeating the Dickinson poem "The Chariot"; this, coupled with a back door program Suzie installed in the Torchwood computers, put the Hub into lockdown, trapping Jack Harkness, Toshiko Sato, Owen Harper, and Ianto Jones inside. With phone assistance from Cardiff police, Jack and Tosh unlocked the Hub using the ISBN on Dickinson's poetry anthology. (TV: They Keep Killing Suzie)