Born in 1886, Golightly was given to a church-run orphanage not long after his birth, and was raised there. As an adult, he found a post as the vicar of a sleepy English village near Eddison Manor. Some time after his fortieth birthday, in 1926, his church suffered a break in, during which he discovered that he was only half-human.
He was, in fact, half-human, half-Vespiform and the illegitimate son of Lady Clemency Eddison and a Vespiform calling itself Christopher. The Vespiform side of his genetic make-up, dormant until then, asserted itself and its psychic bond to the Firestone. The nascent Vespiform used the Firestone to access the mind of Clemency Eddison, and since she was reading a book by Agatha Christie at the time, its personality moulded itself along the lines of an Agatha Christie plot.
At Eddison Manor
Later he killed Miss Chandrakala and Roger Curbishley. The Tenth Doctor and Agatha Christie deduced his part in the murders and expose his true nature. He transformed in front of the guests and chased after Agatha in a rage.
Arnold was killed, in his Vespiform state, by Donna Noble, who tossed the Firestone into Silent Pool. The psychic bond between a Vespiform and its Firestone was so strong that the Vespiform flew into the lake after it and drowned, just as his father had drowned in the 1886 monsoons.
Before discovering his identity, Golightly was a religous man, raised by the Christian fathers while he lived in an orphanage. Upon discovering that he was part-Vespiform, Golightly became a psychotic murderer obbsessed with claiming the Eddison title, partly because of his personality being molded by the words of an Agatha Christie novel. He looked down on humanity and the religion he once followed, declaring it the worship of a "tribal sky god" while considering himself something much more. Despite his psychotic tendencies, Golightly stilled held some compassion within him, evidenced in his dying moments when he severed the mental link between him and Agatha Christie, saving her life.
While his thought processes were based on Agatha Christie's works, Golightly seemed to not hold here in high regard as an author, declaring her works "railway station bookstore romances"
When Donna started to remember her adventures with the Doctor, one of the many creatures she saw was the Vespiform. She asked her grandfather why she could she see a "giant wasp". (TV: The End of Time)