John Kamateros enjoyed imperial favour as Manuel I Komnenos' drinking companion; according to the near-contemporary historian
Niketas Choniates, Kamateros drank wine by the barrel and was able to outdrink any foreign envoy or ruler, while he was a famous glutton as well, eating as if he was starving and able to eradicate entire fields of green peas by himself. Choniates gives an account of Kamateros' rivalry with the
epi tou kanikleiou (keeper of the imperial inkstand),
Theodore Styppeiotes, who was deposed and
blinded at Manuel's orders in 1158/9. According to Choniates, Kamateros resented the fact that, although formally less powerful than himself, who was
logothetes tou dromou, Styppeiotes' office allowed him immediate access to and therefore influence on the emperor. Consequently, Styppeiotes managed to have his own ideas promoted, while Kamateros "saw his demands dispersed in the air like dreams". Frustrated, John forged a correspondence between Styppeiotes and the
Norman king of Sicily,
William II (), which he hid so that it could be discovered easily. Styppeiotes was then charged with treason, lost his offices, was blinded and his tongue was severed. Other authors give different reasons for Styppeiotes' downfall, and the details of Choniates' version have been proven to be inaccurate, at least in their chronology, by the historian Otto Kresten. Nevertheless, as Choniates was a member of the Constantinopolitan bureaucracy and well-informed about its recent history, it is very likely that his information that Kamateros succeeded Styppeiotes as
mesazon (chief minister), before being replaced as both
logothetes tou dromou and
mesazon by
Michael Hagiotheodorites, is correct. As a prominent member of the court, John was also active in the intellectual circles of the time, apparently having himself composed at least two poems in
political verse, and corresponding with scholars like
Michael Glykas and
George Tornikes. John was not a very religious man, and was interested in
astrology, the subject of one of his poems. ==Identification==