Niggle
To the unimaginative, pragmatic world, Niggle appears to be a useless dreamer, an odd little man with little to no societal function, and whose obsession with painting is a bit strange. After a brief conversation, his name is never mentioned again on Earth, and his memory all but forgotten. It is revealed through the events of the story, however, that Niggle's occupation with imagination was not misplaced; instead of being a useless dreamer, he is revealed to be a man who has glimpsed Heaven through art and has been attempting to create to reach this land beyond reality. Unlike the men of the world, he knows what is truly important, and despite his "uselessness" on Earth, he reaches the land of the Mountains.
Parish
Parish, Niggle's neighbor, seems to be an irritating, self-absorbed, earthly man with no regard for Niggle's feelings and no sensitivity to the true, good, or beautiful. Niggle sees him as an interruption to his life, and he cannot wait to be free of him. When they reach the afterlife, however, Niggle finally sees the value in Parish, and vice versa; when the two work together, dreamer and gardener in tandem, they can create something much greater than either of them could individually.
The Journey
For his whole life, Niggle has known that he must eventually undertake a "long journey" (death), an inevitability he finds unpleasant, but an inevitability nonetheless. Instead of dealing with it, though, Niggle merely continues to push preparation for this journey into the future, thinking he'll get around to it eventually. When the time comes for his departure, however, Niggle realizes that he has not prepared; despite many years of time for preparation, he still finds himself ironically ill-equipped to face the journey, mirroring how many humans are unprepared for or even fearful of death.
The Workhouse
When the Porter sees that Niggle has no luggage, he sends him to the Workhouse, a hospital-like place where Niggle is forced to take bitter medicine, work all day, and have no free time. Despite the unpleasant conditions of this Workhouse, it is ironically mild for a representation of Purgatory (a place, according to some Christian traditions, where Christians must go to purge their sins before entering Paradise). Purgatory is typically portrayed as a place of great suffering, where people's sins are painfully purged from them through suffering (see Dante's Purgatorio). In Tolkien's allegorical representation, though, Niggle is nothing more than a little uncomfortable, and he eventually finds his work rather fulfilling.
Niggle's Art
Niggle's artwork, especially the Great Tree, is his life's greatest achievement. When he leaves on his journey, however, his great work of art is taken apart and used to patch holes in roofs, quickly being dismembered and scattered, lost to time and memory. One piece is preserved, a single leaf that someone puts on display in a museum, calling it "Leaf by Niggle." Eventually, however, even that is lost, and Niggle's legacy fades to nothingness. This fate is ironic, as Niggle's Tree exists in a fuller, more realized form in Niggle's Parish, where it is implied it will remain for eternity.
