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 right of the southwestern burial recess, for Michael Tornike (who was Grand Constable for Andronikos Palaiogolos) this saint is represented. It is Saint Mercurius, the Wikipedia has: “Mercurius (224–250 AD) was a Christian saint and a martyr. He was born in the city of Eskentos in Cappadocia, in Eastern Asia Minor (modern day Turkey). His original name, Philopater (variant spellings include Philopateer and Philopatyr), means "lover of the Father". Saint Mercurius is also known by the name Abu-Seifein, which in Arabic means "the holder [literally, owns/possess] of two swords," referring to a second sword given to him, by the Archangel Michael.”

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.