The represented Saint appears to have a strong resemblance to Saint Nestor of Thessaloniki, who was a companion to Saint Demetrius and died as a martyr. Here is his story from wikipedia. http://en.wikipedia.org/wiki/Nestor_of_Thessaloniki
And here is a fresco representing him. http://www.google.gr/imgres?imgurl=http://www.goyouth.org.au/images/stnestor2.gif&imgrefurl=http://www.goyouth.org.au/saintnestormagazine.php&h=335&w=237&sz=68&tbnid=v7tRhEopCbmsrM:&tbnh=90&tbnw=64&zoom=1&usg=__10Ig-mxi-vUKDKZtDgreP9-gi7U=&docid=KLrKPCooUpqbNM&sa=X&ei=HKprUoTHNYnWtAbsjoDwCA&ved=0CEYQ9QEwAw
