That was Orhan, an Ottoman princeling who was a grandson of Sultan Bayezid I, the sultan who was captured by Timur and sparked a interregnum (1402-1413). Orhan's father was one of the various claimants, but he ultimately lost out to Mehmed II's grandfather. Orhan was exiled to Constantinople and the Ottomans paid the Byzantines to keep him detained there. He and his retainers were stationed to guard part of the sea walls on the Sea of Marmara, which was far away from the main action of the siege. After the city fell, he was found and executed.

In the period when the waning Byzantine state coexisted with many Turcoman beyliks, there was a shifting series of alliance between, against, etc. the many Byzantine factions and the various Turcoman states, including the early Ottomans. Not to mention mercenaries. It was hardly unusual that a military engagement of the time and period would have Byzantines and Turcomans on both sides of the fight. To give one example, involving the Ottomans, from Finkel's book Osman's Dream:

Sultan Murad remained in Rumeli following Orhan's death in 1362 until in 1373 he crossed the Dardanelles to campaign in Anatolia, accompanied by John V Palaeologus, who had recently become his vassal. Murad's son Savci and John's son Andronicus chose this moment to rebel against their fathers who swiftly returned home, John to COnstantinople and Murad to Rumeli. Murad had Savci and his fellow rebels killed; Andronicus surrendered and at Murad's insistence was imprisoned and blinded."