A Native in Arthur Ransome's Swallows and Amazons series is a person who is not a member of a crew of sailors, explorers or pirates.
Natives may be adults, relatives or unknown members of the public. All are 'out' of the 'in-group' but natives can be well-regarded (the best of all natives) or enemies (unfriendly natives). In wintertime, the natives tend to wear fur coats and other heavy clothing, and are therefore called "Eskimos". Natives are to be blame for many unneccesary objects and activities such as highways, garbage and noise, but are also the source of civilization (ice cream), which Roger finds important.
The term native may also mean 'associated with worldly concerns' (as opposed to camping , sailing and having fun).
Susan's preoccupations with food preparation and hygiene are often considered native; Captain Nancy thinks (about staying overnight in the Fram) that Susan, she knew, was always a little inclined to take a native view (WH20).
Bill of the Death and Glories is said to sometimes have a "native" demeanour like Susan.