Catherine Louisa Fitzgerald (nee Clarke) is always referred to in Charlie's diaries as 'Aunt Loo'. She was the most frequent of all the visitors to the Payne household in East Dulwich. 'Loo', as she was probably called by all her brothers and sisters and their children, was the youngest daughter of Detective Chief Inspector George Clarke and his wife Elizabeth. Loo appears to have been one of the strongest and most-supportive members of the family. Born on 3 September 1859 in Marylebone she lived for many years with her parents at 20 Great College Street. In September 1877, her father was arrested and charged with perverting the cause of justice, in the first major Metropolitan Police corruption investigation. Loo was called to give evidence in her father's defence, at both the Bow Street magistrate's hearing, and the subsequent Old Bailey trial; evidence that contributed to his acquittal on 20 November 1877. Loo was recorded at the 1881 census as being a teacher, but by 1891, after her father's death, she was functioning as housekeeper to her mother. At the end of the 19th century, she appears to have become the licensee of the Rose and Thistle public house in Camberwell, probably after her mother had moved to live with Loo's married sister Emily in East Dulwich. By 1901, Loo appears to have been employing her brother Harry Clarke as the pub landlord-in-residence, possibly to assist him after he had emerged from prison following a sentence of two years hard labour that he received at the Old Bailey in 1895. However, in 1901, Loo was herself living at 32 Westover Road, Wandsworth.At the time of the 1901 census, Charlie Payne was recorded as staying at Loo's house; whether this was simply a short visit, or a longer-term arrangement is not known.
At the age of 43, on 27 September 1902, Loo married Alexander Fitzgerald at the registry office at St George, Hanover Square. Fitzgerald, a widower, was in business as an 'oil and colour merchant' in Marsham Street, Westminster, and had a 12 year old son, Gerald, from his previous marriage. During 1903 and 1904, the couple were living in Norbury avenue, Thornton Heath, but the marriage was a brief one, as Fitzgerald died from a brain haemorrhage in October 1906; there appear to have been no children from the marriage. Loo was the sole beneficiary of Fitzgerald's will, which left her in a comfortable financial position.
Subsequent details of Loo's life are uncertain. No record of her has been found in the 1911 census, though she may have re-married, and/or excluded herself from the census as part of the Suffragette protest at that time. However,from other diaries and family correspondence, it was clear that Loo was still alive in February 1919. These documents indicate that she clearly regarded her nephew, Charlie Payne, with considerable affection, and showed this in very practical ways. During his time overseas (in Genoa 1907) and on the Western Front in France, Charlie received regular letters from her, and very acceptable parcels containing newspapers, magazines, tobacco and cigarettes.