According to Alexander of Telese, the contemporary chronicler, he was "erudite ... and most prudent in negotiations ... a cleric well-versed in letters, skillful in matters of the world, and possessed of a tenacious and cautious mind."
Guarin was a Norman from France who arrived in Italy not long before Roger was crowned king in 1130.
After the arrival of Roger II on the scene, the insurrection soon collapsed and Guarin took Alife and Raviscanina without opposition.
When, in late Summer 1136, the Emperor Lothair II led a large army down the peninsula, Guarin was sent again to prepare a defence and to force cooperation out of the abbot Seniorectus and the monks of Montecassino.
On 5 January 1137, Guarin demanded their assistance and when refused besieged the monastery in an attempt to seize its treasure and its walls (to use as a fortress against the army of Henry X of Bavaria).