The kings of Kent were not Angles or Saxons – they were actually Jutes, from the north of the Jutland Peninsula. The social organisation of Kent was significantly different from those of the other Anglo-Saxon kingdoms, with only one class of noble as opposed to the two in other kingdoms, while Kentish peasants (ceorls) were also more important than those in the other kingdoms.
According to tradition, the first kings of Kent were the brothers Hengist and Horsa, mercenaries for hire who were invited to Britain by Vortigern to fight against the Picts raiding down the east coast following the collapse of Roman power. In the declining years of the Western Roman Empire it was not at all unusual for barbarian mercenaries to be hired to fight barbarian raiders, so there’s nothing intrinsically unlikely about the tale. It was, however, later embroidered to include details such as Vortigern becoming infatuated with Hengist’s daughter Rowena and signing over Kent to her father in return for the daughter.
It’s only with the long reign of King Æthelberht that historical evidence for the kingdom emerges. The kings of Kent maintained close relations with the Merovingian kings across the Channel, trading widely with them and, as a result, having greater wealth at their disposal than the other kings in Britain. It was this wealth that gave Æthelberht the political clout to be regarded as Bretwalda and it enabled his marriage to a Frankish princess, Bertha. Bertha was Christian, however, and the marriage was contracted on the basis that she would be allowed to remain so.