At the beginning, Groil is just a mystery. Who (or what) is responsible for eating the giant misshapen vegetables that Andrew hides on the roof of the garden shed to keep Mr. Stock the gardener from realizing that they're not being eaten?
Young Aidan Cain is wild to find out who it could be, because he can see that there are trampled bits of grass at the edge of the shed, looking like someone stood there. And it almost makes no sense, because the roof of the shed is over 15 feet high.
Finally, Aidan hears munching noises at night, and races out with a flashlight to discover a very very tall person, wearing a kind of loincloth and a very unsubstantial garment above the waist that looks a bit like a fisherman's net, and with very mussed-up hair. When Andrew's face appears at a window, it turns out that Andrew knew this person, and knew that his name was Groil. It shocks Groil that Andrew has grown so old since he last saw him. Andrew, it turns out, is the one who gave Groil the netlike garment, which at one point was Andrew's old jumper.
Groil is kind, and innocent, and stubborn, and he's poised halfway between a magical kind of world and the world Aidan is living in. He is a giant, but he can make himself smaller and denser when he wants. He observes a lot of what happens around Melstone House, even if he doesn't always understand it.
He is touched and grateful when Aidan decides to enlarge his own old clothes so that they'll fit Groil, and does that so well that they're too big for him at the moment, and have to have the sleeves and trouser cuffs rolled up, which quite delights Groil.
Groil does Aidan several favors too, and is a wonderful part of this novel.