Traditional companion of Sri Guru Nanak Dev Ji; historicity disputed
Bhai Bala
A Hindu Jat of Talwandi who, in the Bala Janamsakhi and in most popular retellings, accompanied Sri Guru Nanak Dev Ji on his travels alongside Mardana and later dictated Guru Ji's life to Sri Guru Angad Dev Ji. The oldest sources do not mention him, and most historians treat him as a figure created by the Janamsakhi that bears his name.
The tradition
The Bala Janamsakhi, which claims to have been dictated by Bala to a scribe in the presence of Sri Guru Angad Dev Ji at Khadur in 1535, presents him as Sri Guru Nanak Dev Ji's childhood friend who went with him on the udasis and lived on into the second Guru's time. This Janamsakhi became the most popular of all the life-stories, and its version of events, including the Kartik birthday, shaped the image of Sri Guru Nanak Dev Ji in paintings, sermons and schoolbooks.
By this account Bala was born in 1466 at Talwandi, outlived Sri Guru Nanak Dev Ji and passed away at Khadur in 1544.[EoS][EoS]
The problem
Bala is absent from the Puratan Janamsakhi, from Bhai Gurdas's first Var, which names Mardana and other early Sikhs, and from the Miharban tradition. The Bala Janamsakhi itself contains material from well after Sri Guru Angad Dev Ji's time and reflects the outlook of a sect opposed to the main line of Gurus. W. H. McLeod's study concluded that Bala was most likely an invention of that text, and the Encyclopaedia of Sikhism records the doubt.
The site treats Bala as part of the tradition, lists the dates the tradition gives, and flags the question wherever he appears.[EoS][McLeod]
What the sources disagree on
Images

Decorated opening page of a Bhai Bala Janamsakhi manuscript. Commons credits the Government Museum, Chandigarh, and gives only a broad date range.
Unknown, 17th to 19th century, uncertain. Government Museum and Art Gallery, Chandigarh (as credited on Commons). Public domain. Source

Bhai Bala reciting the life of Sri Guru Nanak Dev Ji before Sri Guru Angad Dev Ji, as the Bala tradition presents itself. A later artist's imagining, from the Unbound set of Janamsakhi paintings.
Unknown, c. 1775-1800. Asian Art Museum, San Francisco, 1998.58.1 (as credited on Commons). Public domain. Source