Hakeem Nooruddin was the most trusted disciple of Mirza Ghulam Ahmad.
He was the student of Shah Abdul Ghani of the Deobandi school. According to the school,
"After he completed his studios, Shah Sahib told him: "Mian Nuruddin! Books you have already finished; now learn something about remembering Allah". He replied "Sir! I have read the Quran, l have read the Hadith. What's remembrance of Allah besides this?" "Nuruddin! You must have estimated from my lectures on Hadith", said Shah Abdul Ghani, "that l transform the traditional into the rational by the practice of remembering Allah this rational will become perceptible" (http://www.darululoom-deoband.com/english/aboutdarululoom/school_of_thought.htm)
An apothecary by profession, he was from Bhera and was in the employ of the Raja of Kashmir when he moved to Qadian to be with Mirza Ghulam Ahmad, and was his trusted advisor, doctor and friend. It is believed that they had met up first in Kashmir after the death of Mirza's father, circa 1875 (Raees-e-Qadian).
In 1908, he became the head of the Ahmadiyya: according to the Qadiani narration - as the first Khalifa, and according to the Lahori, as the head of the Sadr Anjuman Ahmadiyya, the governing body that Mirza Ghulam Ahmad had left as his successor.
He was also the claimant of many supernatural experiences, although he never claimed direct divine revelation (ilhaam). His administration was marked with internal jockeying for position, a severe reduction in Ahmadiyya numbers and revenue.
In 1914, he died from complications suffered as the result of a fall from a horse. His death led to the first schism in the Ahmadiyya.
His thoughts and beliefs were a mixture of Sufism and Deobandi doctrine. He shifts easily from literary to metaphorical and back, as can be seen by this collection of comments on the Quran: Aqwal-e-Zareen fi Irshad Nooruddin (12 MB PDF). When he was to be married to a lady from Ludhiana, this conflict also comes up in his correspondence.
Another influence on him was Sir Syed Ahmad Khan. Through his correspondence with Mirza Ghulam Ahmad (from Maktoobat), Nooruddin had the idea for the original claim of 'matheel-e-maseeh' (like the Messiah) and advised the Mirza against claiming to be the real Messiah, which the Mirza did anyways.
His loyalty came from the Sufi side of his thought. Whereas Deobandi scholars like Batalvi could not compromise their intellectual integrity and distanced themselves from the Mirza over time, Nooruddin would continue to guide the Mirza and be guided by him. He is believed to have famously said, 'I did not abandon Mirza even during the Muhammadi Begum saga, why would I abandon him now?'
Mirza Mahmud Ahmad of his allegiance.