Fresco of Saint Demetrios from Dečani church, Monastery St. Theodore Tiro (Tyro), Visoki Dečani, Serbia.

This is a very clear depiction of a high-ranking soldier from the early 14th century and probably the larger part of the 13th. It is from Dechani monastery in Serbia and is from c 1320-40. It depicts St. Demetrious killing the Bulgarian Tsar Ioannis.
Source: Roman Army Talk

Referenced on p.33, Byzantine Armies AD 1118-1461 by Ian Heath & Angus McBride
An early 14th century portrayal of St Demetrius, patron saint of Thessalonika, killing Tsar Kalojan of Bulgaria (1197-1207). Kalojan's armour closely resembles that described by Theodore Palaeologos in 1326 as typical harness of the Westernised Byzantine soldiers to be found in Greece, comprising pourpoint, mail corselet, gorgeré (hood or collar?), cuirie, gambeson, 'greaves' (probably mail chausses), cuisses, and helmet. Kalojan's quilted cuisses have plate knee-guards attached.

See also a Fresco of a warrior saint from the north choir of the Dečani church nave, Monastery St. Theodore Tiro (Tyro), Visoki Dečani, Serbia, 14th century.
Fresco of the White Angel and Sleeping Guards at the Sepulchre in Mileševa Monastery, Serbia, c.1240.
Byzantine Illustrations of Costume and Soldiers
Other 14th Century Illustrations of Costume and Soldiers