Sitalkes is a Getae. The Getae were one of the most war-like of the Thracian tribes, and were either Scythian in origin or had learned many of the Scythian life-ways from repeated contact. At least one archaeologist has noted that western Scythian graves and eastern Getae graves are indistinguishable.
Despite this (or, perhaps, because of it), they were inveterate enemies. Kineas takes Sitalkes as a prisoner and makes him a slave — the ruthless way of the world. Once Sitalkes is removed from his home and tribal lands, he is loyal to Kineas because he has no other choice — a situation the reader will see repeated over and over because it seems to have been so common in the era.
But Kineas does free Sitalkes, at least in part because Kineas is impatient of slavery. To be honest, I’m not sure this is an historically defensible position. There seems to be very little outcry over the treatment of slaves in the ancient world, but it seems to me that any student of Plato must eventually find the imposition of slavery at least distasteful.
Once free, Sitalkes becomes Kineas’s bodyguard, along with the giant Keltoi Carlus (actually a Marcomanni) and Hama.