Wikipedia
Leucosyri, or Leucosyroi (Singular: Leucosyros), also known as Leocosyrians or White Syrians, are an ancient tribe that existed in Anatolia. They were eventually hellenised. The name "Leucosyri" derived from the Greek name Λεύκοσυροι or Λεύκοσυριοι meaning White Syrians. Possible theories of the origins of the Leucosyri is that they may have been related or an offshoot of the Persians who also inhabit Anatolia as the same time as the Greeks. Whether they are a subgroup of the Syro Hittites or Iranians or some form of relation, it's been believed by most scholars and people that the Leucosyri are of Iranian and Syro-Hittites origin with possible mixed Anatolian blood.
They are referred as 'Cappadocians' by the Persians, later 'White Syrians' by Greeks. They have mixed Iranian, Syro-Hittites and Anatolic origins.
It has been believed that the Leucosyri lived in Pontus, Cappadocia, Cilicia and parts of central-west Asia Minor.
The Leucosyri of Pontus mainly lived in the mountainous highland of Pontus near Armenia Minor with considerable amount of people living in the interior of Pontus (the coastal area). During the reign of the Kingdom of Pontus, they may have been one of the Iranians of Pontus and Taurica (another traditional area with a large population of Pontic Greeks), to have contributed to Pontus' Iranian and Anatolian influence of the ancient Pontic population. By the time of Justinian I, the leucosyri have been hellenized.
During the Mithridatic Wars, the Leucosyrians were recruited as mercenaries into the Pontic army to fight off the Roman army of Sulla and later Pompey.
The Leucosyri have also lived in Cappadocia amongst the Greeks, Persians, Armenians, Diauekhi, Taochi and other Anatolian tribes such as the Galatians. Like the Leucosyri of Pontus, the remaining Leucosyri assimilated into the Cappadocian Greek population.
It have never been clear of the background of the Leucosyri as history had mention them a few times. The religion have never been clear of what deities the Leucosyri worshipped. Leucosyri may have worshipped Iranian, Anatolian and to some extent Aramaic gods and goddesses throughout eastern Anatolia. It is then known that throughout the late Roman Empire they were introduced to Christianity.