Emily Holroyd operated out of the Torchwood Institute Cardiff branch, known in her day as Torchwood Cardiff and later as Torchwood Three. She was in a relationship with fellow Torchwood agent Alice Guppy.
Circa 1899, she investigated and helped to recruit Jack Harkness to Torchwood, having already suspected both that he could not die and that he also might well know how to find Torchwood's enemy, the Doctor (Jack did not actually know himself how to locate the Doctor). (TV: Fragments)
Around 1899, she was contacted by William Mayhew about the HMS Hades. When Jack went in her stead and was captured, Torchwood began investigating. She and the rest of Torchwood infiltrated and destroyed the HMS Hades after it was found to be a lab for experimenting on aliens. (PROSE: The Baby Farmers)
Behind the scenes
The series 2 version of the Torchwood website showed a diary entry describing how Alice (unnamed in the text) became a member of Torchwood Three. It said that Emily, who had previously been married, had a husband, whose situation in late 1898 was "complex".
After the death of one of Emily's team, Alice, expecting a reprimand for having wounded Mrs Hailsham in the laundry room on 12 September 1898, was given a contract on the 13th to be employed as an Investigations Agent for Torchwood by the governess and have her charges dropped by signing it.
The following day, Emily collected Alice for the trip to Cardiff. She informed her of the death of Emily's previous partner in Torchwood, who had succumbed to a "mutative infection", becoming the new queen of a snake-like race. Emily was "obliged" to burn her partner to death. She also told Alice what the Torchwood Institute was about.
According to the diary, Emily brought Alice to Cardiff on the 15th, and Alice began her patrol on the night of the 16th. 
According to a morgue stock take by Owen Harper on the series 2 version of the website, bay 5 of the morgue held contents unknown to Owen, which was locked by order of Emily and under "strict instructions" not to get it open until it was ready. Owen noted that at the time of the check, the bay was due to be opened up "in a few months".  This refers to the events of TV: Exit Wounds, where a future version of Jack Harkness insisted on being frozen in the morgue undisturbed until a certain point in time to confront his brother Gray. He was rescued from a millennia-long torture of being taken back in time and then buried alive to resurrect endlessly due to his immortality. However, this Jack had already outlived the Victorian-era Torchwood and was now native to the 21st century, so he chose to be frozen to avoid disrupting history and possibly running into his younger self.