warning: plot spoilers may follow
Arthur is a theoretical physicist and astronomer who was in love with Elizabeth "Liz" Pennykettle when they were both younger. However, Arthur was unaware at that time of Liz's special relationship to the dragons and Guinevere, and when he hears about Liz's "pregnancy" with Lucy from "Aunty Gwyneth", he believes there is another man involved and leaves. We discover him many years later, in the book Fire Star, now under the name of Brother Vincent and sheltering in a monastery on Farlowe Island.
Later in the book, Arthur is attacked by one of the mysterious thought forms, known as the Fain. While many of the species are benevolent creatures, this one is not, and his ordeal with the 'Ix' renders him almost completely blind, instead of destroying his mind as planned.
Arthur later discovers the unusual circumstances of Lucy's birth and goes to live in the Pennykettle household once again. He proposes to Liz, and this causes some rift at 42 Wayward Crescent when Liz reveals to Lucy openly that they may try for a normal human child. Whenever Arthur comes into physical contact with Bonnington, their cat, who is hosting a non-hostile Fain, he is able to see again for as long as he is touching the animal. At the end of The Fire Eternal, it is revealed that Liz is now pregnant with Arthur's child, a boy, though as she was poisoned with obsidian by a controlled Lucy, the child could die.
In Dark Fire, the boy, called Joseph Henry decides that he would rather be a dragon. so he transfers his mind into the body of the once lifeless Gwillan. Joesph Henry survives.
In Fire World he is called Harlan and still a physicist. He is banished ton the Dead Lands for the accidental death of Mr.Henry.
In the Fire Ascending Arthur very little is said about him. He is beside Liz when she dies.