Who is The Boggart, the Brownie, the Creature from The Field Guide and what is their importance?

The reader doesn't meet the boggart until the very end of the novel, when a pencil-sized, old man appears to the children. Initially, he makes his presence known by a series of scratchy noises in the wall of the old Victorian house where the Graces live. Over time, his nest is discovered by the children in the kitchen. The boggart then writes a nasty note in the dust that Jared sees in the library. He is described as mischievous, buthe is also open to being mollified by such things as the new nest that Jared builds for him. At the end of the story, the boggart appears and makes clear that the children should stop trying to learn about faeries and other mystical creatures in the house and area.