She refers to herself as Louise - Lou seems to be a sort of pet name that Liz calls her
Talk about it here.
Although Louise did assist in Liz's investigations, her responsibilities were largely at Ashley House, doing research and administration and making tea. Although she knew of Liz's research at Cambridge, she was unaware of her career at UNIT, being told that it was "hush hush".
Louise assisted Liz with her investigation into the murders around the Hawthorne clinic, helping her to convince Patsy Haggard not to let the case go to Scotland Yard. She told Liz who Peter Russell was and surmised that he kept the clinic open because of his family ties after Liz discovered his connection to Jeremiah O'Kane.
Whilst Liz visited Russell and Colin Dove, Louise asked Beatrice Hearst and Julie Gilchrist about William Bruffin's murder. From her, she obtained a transfer record showing that Dove had moved Patient Zero to the hospital at the same time that the killing began. (HOMEVID: The Zero Imperative)
In 1995, after her father's death, Liz took a few weeks of compassionate leave. During this time, Sir Richard Stevenson was replaced as the minister in charge of P.R.O.B.E. with Brian Rutherford, who saw no need for the organisation and transferred Louise to a different department, leaving Liz as its only member.