Billy Lewthwaite is Mrs Lewthwaite's younger son and Sammy Lewthwaite's younger brother.
He does odd jobs, eg driving: when Cook says that Billy will be needed to meet the Ds at the station Nancy says they are going to sail across and if they have too much luggage to carry Billy Lewthwaite can fetch it in Rattletrap tomorrow (PM1).
When the GA wants to see for herself Cook fetches Billy, who took the old yachting cap that he liked to wear (with his blue coat) when he was being a chauffeur (PM23,25)
Billy lives with his mother and appears to be single. When he is asked to drive the GA, he is digging in the potato patch behind his mother's cottage .... and was glad to have a chance of driving old Rattletrap as a change from digging potatoes (PM23).