Portuguese had ruled Cochin for nearly 160 years between 1503 and 1663 before the Dutch invasion. Although the capital of Portuguese India was shifted from Cochin to Goa in 1510, Cochin continued a favourite destination for the Portuguese and many Portuguese families chose to stay on in Cochin, soaking in the sun and sand, gossiping and leading their exclusive fidalgo life. (Fidalgo literally means 'son of somebody' and refers to nobility.)
Juliana was born in 1658 in Cochin to Agostino Diaz da Costa and his wife. She grew up as a frolicsome young girl, playing on 'the sandy beaches, where my sister and I could run with the waves lapping our feet'. When she was five, fate struck in the form of the Dutch who invaded Cochin in 1663. Just before the Dutch attack started, the da Costa family managed to flee to Goa, although they lost all their baggage in a ship wreck. The family then decided to try their luck in another Portuguese enclave, Calcutta, but by the time they reached there, Portuguese there had earned such a bad name through their indulgence in piracy and slave trade that  the conditions were not considered favourable for their relocation to Calcutta.
It was then that the da Costas decided to move down to Agra where the father had been invited to attend on the Emperor. It was here that Juliana got to know the doctor who attended to the Mughal emperors whom she married later. Juliana herself was adept at home remedies, having picked up some from her stay in Goa and from Garcia de Orta's book Colloquios published in 1563(We in Kerala know much more about Hortus Malabaricus which was published more than a hundred years later in 1678. Garcia was himself a medical doctor - unlike Van Rheede who depended on local vaidyans like Itty Achuthan)
Juliana got to know the royals closely through her husband and even had an audience with Aurungzeb, thanks to the influential Jesuit priest Fr. Magalhaes (a colourful character who worked assiduously for promoting Society of Jesus in India and China). Juliana recorded faithfully the experience of an audience with the Alamgir who had a reputation for being brusque and curt. 'The old emperor was sharp, but I was amazed at the amount of time he spent talking with me. He asked me a great deal about the Malabar region, of the Portuguese interests, and of the Deccan interaction with the Portuguese'.
Juliana was soon appointed as Superintendent of the Zenana, looking after the women in the Palace and teaching the young princes and princesses. Juliana soon came to be known for her piety and her ability to work miracles - putting out fires with consecrated palm fronds and curing illness through prayers. She was particularly close to Prince Muazzam who carried the title Shah Alam and was later to be crowned as Bahadur Shah in 1709, after killing his brother. 
 Juliana continued in the Mughal Court even after the death of Bahadur Shah in 1712 and continued to serve the Mughal household with her advice, prayers and cures. Farukhsiyar ascended the throne in 1713 after another bout of internecine blood-letting, but Juliana not only survived the intrigues of the powerful Sayyid brothers who had the Emperor under their control, but even had powers to get the Emperor to issue firmans.
British colonial historians have been asserting that it was the English surgeon, William Hamilton who had cured Farukhsiyar of a painful carbuncle and obtained a firman  for trading without duties. But, apparently, it was Juliana who had cured the Emperor with her herbal concoctions (and a liberal dose of Christian prayers). She records that she had got firmans out of Farukhsiyar not only for the Portuguese, but even for the English traders!
Mohammed Shah
courtesy: wikipedia
The crowning glory of Juliana's days in the Mughal Empire was in 1719 when she was asked to physically crown the new Emperor, Mohammed Shah (Rangila)! The day she chose for this was, of course, the day of St.John the Baptist, her Patron Saint. She wrote: 'At mid-morning today, I , Juliana Diaz da Costa, actually crowned the emperor! I carried the crown and placed it on the head of Prince Mohammed Shah'.
Donna Juliana (she had been conferred the title for her services to the Church and the Jesuits) continued in the service of the Mughals. A letter written in 1727 testifies: 'The Chief Surgeon of Bacaim is in the Court, who has been called to look after the mother of the king. The treatment is pending the arrival of Donna Juliana to the palace, to touch and give medicines to the patient with the help of the Surgeon mentioned'.
Juliana passed away in 1732 and was buried in Agra in an unnamed grave! Thus ended the saga of the girl from Fort Cochin who wielded great influence in the Mughal Court during an era when heads around her were rolling in the relentless wars of succession.
Source : Forgotten (2010) by Bilkees I. Latif, Penguin Books