As the eldest son of Emperor
Alexios I Comnenos, John had the status of
Porphyrogenitus, "born to the purple." He had an older and well-educated sister,
Anna, who had hoped that she and her husband would rule after Alexios' death, but it was not to be. Her brother became Emperor John II Comnenos, the second emperor of what is called the Comnenian Restoration that lasted more than a century.
He was known as John the Good or John the Beautiful, but this was a reference to his character, not his physical appearance (he was short, and his eyes and hair and complexion were so dark that he was also called "the Moor"). He was pious, dignified, and (notable for a medieval ruler) known to be faithful to his wife (Irene of Hungary, venerated as a saint). A just ruler, supposedly he never had anyone executed outside of what the law demanded.
He was also a good general and strategist. Unlike his father and other emperors, he did not use his family to fill high positions (which in the past created opportunities for family members to consider conspiracies and coups). He appointed several men from outside his family connections, distributing (and diluting) power among several different clans.
His closest friend and advisor was John Axouch, who was himself so moral that, when John offered him all the property of John's sister Anna (because John had uncovered a conspiracy of which she was a part), Axouch declined. Axouch was a Turk who had been captured at the Siege of Nicaea and given to Alexios as a gift. John appointed Axouch as the megas domestikos, the Grand Domestic, making him commander-in-chief of the Byzantine armed forces.
John II's policies did lead to resentment among his family. His younger brother, Isaac, was conspiring against John, and in 1130 when John was about to have him arrested for it, Isaac fled first to a Danishmend emir, then to the Seljuk Sultan of Rum, and also to the Prince of Armenia. Isaac was apparently seeking aid from these rulers. Isaac had some support in Constantinople, and John had to hurry back home from a campaign upon learning that there was a plan to put Isaac on the throne. A reconciliation of the brothers in 1138 was short-lived: a year later Isaac was exiled to Heraclea, on the coast of the Black Sea, far from Constantinople.
During his reign, John personally led all military campaigns to expand territory or put down rebellions. He is regarded as the most successful of the Comnenian emperors. His death was accidental. While hunting wild boar in Cilicia in southern Turkey, he cut his hand on a poisoned arrow. We assume he did not bother treating the cut, which became infected; he died several days after the incident, on 8 April 1143. Of his surviving sons, he named the younger, Manuel, to be his heir rather than the older Isaac. Isaac was known to be irritable, while Manuel had distinguished himself on campaign. John Axouch argued for the older Isaac, but was a faithful supporter of Manuel when the choice was made.
There is another theory offered to explain the choice of Manuel over Isaac, and it is the odd AIMA prophecy. This is a quirky one, which I'll save for tomorrow.