Alwin I rose to prominence when he successfully stopped the Romans, from conquering the British Isles to become part of the Roman Republic. The Celts responded by choosing Alwin I as the Celtic super-king. His descendants would have also stopped Claudius' conquest of Britain in 43 AD, which would otherwise prevent the timeline from diverting. It is unknown how much his dynasty has ruled until the coming of Alfred the Great, (historically, Claudius managed to conquer Britain, up until Scotland, starting from 43 AD to around 54 or 55 AD).
According to Lelouch, his real historical existance in contested, meaning he can be nothing but a lie of the Britannia's propaganda.