To the left of the southeastern recess (as can be seen of the overview picture) is this saint. 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.

I was informed this is St. George. From the Wikipedia: "Saint George (Greek: Γεώργιος Georgios; Latin: Georgius; AD 275–281 to 23 April 303), according to legend, was a soldier in the Roman army who later became venerated as a Christian martyr. His parents were Christians of Greek background"

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.