Was quasimodo real?

The character of Quasimodo is indeed based on a real person; a hunchback who lived in Paris in the 1820's and was seen working in the cathedral.
The discovery of the real Quasimodo, or, more likely, just Victor Hugo's inspiration for the character, was made by the British archivist Adrian Glew.

Glew was studying the sculptor Henry Sibson's autobiography and suddenly came across a description of a hunchbacked man working in the cathedral, chopping stone.

This hunchback can very well have been Victor Hugo's (the author of the novel "The Hunchback of Notre Dame", in French "Notre Dame de Paris") inspiration for the hunchbacked main character Quasimodo. It is commonly known that Hugo very often came to the cathedral to seek inspiration for his novel, which he started writing in 1829.

Henry Sibson never spoke to the hunchback, and the real Quasimodo's name remains unknown. It is also yet to be discovered whether the hunchback also worked as a bell ringer, or if he was just hired to help with the renovation.

Bonus info: Henry Sibson's autobiography mentions the name of another sculptor, Trajan. The Parisian archives confirms Trajan as a sculptor in the 1820's, which strengthens the reliability of Sibson's autobiography.
Furthermore, Trajan might have been another one of Hugo's inspirations: In an early version of Hugo's "Les Miserables", the main character Jean Valjean is called Jean Trajean, maybe inspired by this other sculptor from Sibson's autobiography.