Mithrenes was a general of the Persian army fought by Iskandar. After his defeat to Iskandar, Mithrenes became one of his followers. After death, he was sublimated as a Heroic Spirit, one of the many soldiers able to be called by Iskandar's Noble Phantasm Ionioi Hetairoi due to their shared bond as soldier and retainer.
Like all of his fellow comrades in Ionioi Hetairoi, he is unquestionably loyal to Iskander and always supports his cause.
Mithrenes is summoned outside of Iskandar's Reality Marble during the fight with Gilles de Rais' summoned monster to act as a scout to relay information between Iskandar and Waver Velvet to cordinate the usage of Saber's Excalibur against the monster.
When summoned as a Servant of Ionioi Hetairoi, Mithrenes does not have a Servant class. He has E- Independent Action, so he can only exist for "30 turns" upon manifestation. Ionioi Hetairoi cannot manifest Noble Phantasms for the summoned soldiers, so he lacks any such armament.