ATTENTION: SPOILERS BELOW !
Very well. Here is the "backstory," as we currently understand it. (Note that this is an organic entity, still unfolding.) This page will only make sense to you if you have read the mass-market edition of Cabinet of Curiosities, or at least page 580 of the mass-market edition, or certain later Pendergast novels. And remember, the information below will reveal all— it isn’t called a ‘spoiler’ for nothing.
You have been warned!
Though Enoch Leng hated humanity with a passion, he— with a capriciousness typical of mankind— grew to feel an affection for one person, and one person alone. That person was an abandoned waif named Constance Greene, younger sister of one Mary Greene of Water Street. (Mary was an early victim of Leng’s whose remains are discovered by Nora Kelly and Agent Pendergast early on in Cabinet of Curiosities.) Leng would have met Constance in his "work" at the Houses of Industry in lower Manhattan, where orphans were once housed. Perhaps it was something in Constance herself, or perhaps it was a vestigial sense of guilt within Leng, but for whatever the reason he grew fond enough of this last surviving member of the Greene family to take her into his home and, in effect, adopt her. This would have been around the year 1882, when Constance was six. The life-prolonging elixir that he used on himself he also gave to her. This served to greatly retard her aging.
For over a hundred years, the two lived a reclusive existence together in the slowly decaying mansion on Riverside Drive. Although Leng eventually grew tired of life and stopped taking the elixir, he continued to administer his (now-synthetic) formula to Constance. Consequently, she aged very slowly: by the end of the twentieth century she was, physically, no older than a girl of nineteen. However, over those many years, Leng had lavished his deep erudition on her. Constance had the run of Leng’s collections and his extensive library, and so— though in appearance she had barely reached adulthood— mentally she had achieved several lifetime’s worth of learning and experience.
And then the unexpected happened. Anthony Fairhaven, in his desperate search for ‘The Surgeon’ and his secret arcanum for life prolongation, tracked Enoch Leng to his mansion on Riverside Drive. Leng, now physically an old man, was powerless against Fairhaven’s feverish brutality. Tired of life, Leng died amidst Fairhaven’s interrogations without ever revealing his secret formula.
Throughout all this, Constance Greene remained undetected. Shocked beyond measure by the sudden, violent assault on the only person she cared for and on the home she had not stepped out of for more than a century, she retreated into the mansion’s secret honeycomb of passages, into the deepest levels, where the rambling sub-basements of the edifice gave out onto the banks of the Hudson River. From concealed spots known only to her, she watched— powerless to act— as the climactic confrontation told in the last pages of Cabinet of Curiosities played out.
Ultimately, Fairhaven died and Special Agent Pendergast— the last surviving relative of Enoch Leng— claimed the mansion and its fabulous collections for himself. After finding and burning Leng’s formula for life prolongation, Pendergast departs from New York for a time, leaving the house in the hands of his acquaintance, Wren, who is given the job of cataloguing its cabinets of curiosity.
Meanwhile, as the weeks pass, Constance watches. She lives on scraps of stored food and on nocturnal forays out onto the twisted banks and darkened streets adjoining the Hudson. The terrifying events that have shattered her long-sheltered life are so traumatizing that for months she keeps her presence in the house a secret. Wren, hard at work in the basement collections, is in fact aware of a feeling of being watched, and of occasionally hearing the patter of retreating footsteps— but it is not until Pendergast is preparing to return to New York (as described in the final chapters of Still Life With Crows) that Constance at last plucks up the courage to reveal herself to him.
When Pendergast learns about Constance’s existence— and when, in addition, he learns her remarkable history— he realizes he has no choice but to take her under his wing. Not only was the girl uniquely ill-equipped to set out on her own, but along with the house, Pendergast had inherited Leng’s familial guilt as well. And so, though she will probably remain a secondary character, Constance will become Pendergast’s ward— as well as research assistant, social secretary, amanuensis, seer: the beautiful young girl with the eyes that seem to possess, unaccountably, a depth of almost limitless experience.
This is not the end of the story, of course. There is more--much more--in the novels that make up the 'Pendergast trilogy:' Brimstone, Dance of Death, and The Book of the Dead, as well as later Pendergast novels.
