The Hunchback Assignments by Arthur Slade

From chapters.ca:
There are rumors that a hunchback infant in a gypsy freak show has the power to transform his appearance. This comes to the attention of Mr. Socrates, a member of the shadowy Permanent Association, who decides to take the boy back to England and raise him for his own purposes. Naming him Modo, Mr. Socrates keeps the boy indoors while putting him through training to be a secret agent. When Modo turns 14, his education is complete. He is taken to foggy, polluted London and abandoned, penniless, to test his skills. But Modo is resourceful, and he finds a way to get by, keeping to himself...until one day, when the beautiful Octavia Milkweed knocks on his door. Soon, with the help of Mr. Socrates, they find themselves uncovering a sinister plot being carried out in the very sewers beneath their feet.