The guidebook I used as a reference explains how the Byzantines saw martyrs and warrior saints as holy people who could intercede to God. So they were often represented on the lower walls of chapels.

To the left of the southwestern burial recess, for Michael Tornike (who was Grand Constable for Andronikos Palaiogolos) this saint is represented. I was informed this is Theodore Stratelates. From the Wikipedia: " (in Greek, Στρατηλάτης, translated as ("the General" or "Military Commander"), also known as Theodore of Heraclea, is a martyr and Warrior Saint venerated with the title Great-martyr in the Eastern Orthodox Church, Eastern Catholic and Roman Catholic Churches.

There is much confusion between him and St. Theodore of Amasea and they were in fact probably the same person [citation needed], whose legends later diverged into two separate traditions."

Warrior saints wear short tunics beneath armour and often wield a spear, martyrs have no armour and wear martyrs' robes. Some saints are depicted as just portraits, in medaillons.