Because the covered figure outline is gradually becoming visible as the outer colour wash becomes transparent with age I strongly suspected it was either Branwell Bronte himself, the common conclusion, or, because of what appears to be a white cravat worn by the figure, Branwell's father the Rev Patrick Bronte. Out of curiosity and some free time I engaged in lengthy red and yellow overtone and contrast enhance experimenting with the GIMP image editor program. What I discovered was very unexpected.
The previous cravat "cup" shape located in the space beneath the right eye of Charlotte and the left eye of Emily was revealed to be the face of a short African male standing in the background and wearing what appears to be a high standing white ceremonial headdress which has a band of skull like faces around it a third of the way above the base. These "faces" are easier to see when the painting is converted to a grey scale image and enlarged.
I checked the book "The Art of the Brontes" by Christine Alexander and Jane Sellars [Cambridge University Press 1995 ISBN 0521 43841] and found several Branwell paintings and drawings with echoes of this African face.
Page 293 has a c1829 sketch drawing of a possibly African man with a balloon shaped head smoking a pipe. When this pipe is covered by a readers finger the similarity with the figure in the three sisters painting is more obvious.
Page 342 has a c1841- 1842 Branwell note book sketch of two swarthy skinned boxers. The taller one is wearing a high peak head covering.
The African face is not the only mysterious figure in the painting. British Newspapers Archive have a scan of one of the earliest known media photos of this painting taken soon after it was first discovered. The Emily section looks different to the version we know today. Her face looks fuller and her hair thicker and looser but this may due to the low resolution of the image blurring the separation between front and back in a grey scale setting. As well the fold that went strongly through Emily's right eye in The Sphere image has been lessened so that its less noticeable in the well known National Portrait Gallery version.
A figure can also be seen lurking in the shadowy background behind Emily's face. This figure has his right eye closed or missing, his right ear is vaguely visible, he seems to have some type of beard and a couple of his upper front teeth are showing while his face has an agitated, wary expression and he could have a ruffle around his neck. This figure is not visible at all in later variations of the painting [creased or uncreased]
There also could be a male figure with a drooping moustache to the right of Charlotte in The Sphere photo. He could be an exhibit observer reflected in the glass protecting the painting if this was not removed before the image was recorded.
![Bronte sisters painting by Branwell Bronte [restored] Wikimedia Commons image](https://commons.wikimedia.org/wiki/File:The_Bront%C3%AB_Sisters_by_Patrick_Branwell_Bront%C3%AB_restored.jpg){kind=link}