The Bala Janamsakhi is the most widely circulated life of Guru Nanak Dev Ji and is attributed to Bhai Bala, described as a companion of the Guru from his own village who travelled with him.
Scholarship has been sceptical of the whole thing since the nineteenth century. Bhai Bala is not mentioned in the Puratan tradition, not in Bhai Gurdas Ji's Var, and not in any early source, and the Bala Janamsakhi contains material that points to a much later composition, probably by the Handali sect.
The text nonetheless shaped popular understanding of Guru Nanak Dev Ji more than any other, because it was the version that circulated in cheap print.
The paintings that show a third figure beside the Guru and Bhai Mardana Ji are showing Bhai Bala, and are showing a figure whose existence is not established.
This is included here because a project about what is known has to say when the most familiar version of something is the least supported.
He appears in one janamsakhi tradition as a lifelong companion of Guru Nanak Sahib Ji and is absent from the others, including the older ones.
The Bala janamsakhi is the most widely circulated of them and the most textually suspect. Scholars from the Singh Sabha period onward have questioned whether he existed at all.
The argument matters because the Bala tradition is where a great many popular sakhis come from. Rejecting the source does not take the stories out of circulation.