The Stig - probably taken from the book 'Stig of the Dump'
Stig is a caveman. He lives at the bottom of the old quarry close to Barney's grandparents' house. Since the quarry isn't used any more people throw all their old junk away down there. So it is rather an interesting place to build a den.
How does Barney find Stig? Well, he just falls over the edge of the quarry and tumbles down through the roof of Stig's den. When he looks round, there's Stig, with his shaggy black hair and bright black eyes.
Barney and Stig get on rather well together. They have to manage without language, of course, but that doesn't seem to stop them. Stig's den is a brilliant place built out of discarded rubbish
Stig is Barney's secret friend, not because Barney doesn't tell anyone, but because no-one really believes that Stig is real. They have a great time, improving Stig's den, collecting firewood, going hunting, and even catching some burglars who break into Barney's grandparents' house. It's really a collection of short-story adventures.
...although it could also have rallying connotations