The victim, a girl in her mid-teens, was raped on March 7 after she left a block of flats in Joyce Avenue, Edmonton, at 8.55pm.
Police were called to Fore Street where a woman, who identified herself as “Sandra”, spoke to the victim and comforted her.
The woman then left the area when the police arrived, on the 149 bus heading towards London Bridge.
She is described as a black female of medium build, wearing a black coat, a long red skirt, and carrying a plastic bag.
Following an investigation, a 29-year-old man from Haringey was charged with one count of rape and sexual activity with a child.
Detective Constable Rebecca Mussington, from the local policing team in Enfield, said: “This woman spoke to and comforted the young girl and we really need to speak to her.
“She may not realise it, but her evidence could be crucial. This is a very serious allegation of sexual crime against a young girl and I would appeal to this lady, or anyone else who has any information to come forward and tell us what you know.
“You can always call Crimestoppers if you do not want to give your name, but however you want to do it, please call.”
Any witnesses or anyone with any information is asked to call police on 101 quoting CAD6590/06Mar. To give information anonymously contact the independent charity Crimestoppers on 0800 555 111.