Bhai Mardana Ji was a Muslim mirasi of Rai Bhoi di Talwandi, ten years older than Guru Nanak Dev Ji and from the same village. He played the rabab and travelled with the Guru for the whole of the udasis, some twenty years and more.
The relationship as the janamsakhis record it is not one of disciple and master only. Mardana Ji argues, complains of hunger, gets tired, and is answered. He is the one person in the accounts who talks back.
Three couplets attributed to him are in the Guru Granth Sahib Ji, under Bihagra.
He is the origin of the rababi tradition, families of Muslim musicians who sang kirtan at the Harmandir and at gurdwaras across the Punjab for four centuries in unbroken descent from him. That line ended at the Golden Temple in 1947.
That a Muslim musician's family sang the scripture in the central Sikh shrine until Partition is a fact about Punjab that almost nobody outside it knows, and a good many inside it have forgotten.
He was from Talwandi, of a family of mirasis, and he travelled with Guru Nanak Sahib Ji for most of the udasis, which is nearly the whole working life of both men.
Three shloks attributed to him are in the Guru Granth Sahib Ji. He is the only companion of the first Guru whose words are in the volume.
The rabab he played, and the families who inherited that role, are the origin of the whole kirtan tradition. That the first Sikh musician was a Muslim is stated in every account and acted on in almost none.