For starters, you're probably calling to mind this image of Quasimodo. So wholesome. So self-actualized despite his crippling deformities and hermit lifestyle. After all, he's got those sassy gargoyle friends, and a bedroom view other Parisians would kill for. Life's not so bad!
This is probably the biggest liberty Disney took, and for good reason: literary Quasi is kind of a fuck. Consistently described as angry, distrustful, and apt to use his brute strength on anyone who gets in his way, he's hardly the gentle mutant of 1996 imaginations. Given his inauspicious start, there is at least reason for the rage; Hugo explains that the 4-year-old Quasimodo was wrapped in a sack and deposited on a public offeratory bench where unwanted babies could be claimed by anyone who wished to take them. (What? This was a thing?) Unfortunately for Quasi, the repulsed observers were pretty sure he was (literally) the spawn of Satan, and were edging towards a mercy killing before a priest intervened. Honestly, things probably would've gone better for everyone if he hadn't, but that wouldn't be much of a book.
The priest in question is Claude Frollo, a somber scholar who entered the clergy to devote more time to his twin passions, reading and theology, and to avoid marriage-centric secular life, which he views with disdain and indifference because he's never had any sexual impulses. Yeah... that'll last.
So Frollo takes pity on the baby-bench reject and decides to raise Quasi as his own, and the child becomes the bell ringer of Notre Dame. Too grotesque to mingle much in public, Quasi forms a deep connection with his bells, to which he has assigned names and personalities. The downside: their continuous cacophony soon renders him completely deaf. And he only has one (bad) eye, so he's also nearly blind. And there are no talking gargoyles. I'm starting to get depressed.
Meanwhile, outside the cloister, some other characters are milling about. There's Esmeralda, the sexy teenage gypsy who earns money by dancing and performing tricks with her pet goat in the square outside the cathedral, and Gringoire, a penniless poet who soon falls in with her crew. There's also Phoebus, an army captain who enjoys sleeping with any girl who will stay still long enough. Over the course of the book, every major male character attempts to get into Esmeralda's pants. Surprise! It doesn't end well.
Bachleor #1: Gringoire. After Esmeralda intervenes to stop him from being murdered at the hands of a vagabond mob, the head gypsy in charge decides he'll spare Gringoire if Esmeralda marries him. She consents, on the condition that the marriage be platonic. Gringoire briefly mourns the irony of being blue-balled by the hottest exotic dancer this side of the Seine, but decides it's better than being hanged.
Bachelor #2: Phoebus. He's young, he's hot, and he's a high-ranking officer. He's also betrothed to some socialite, but that's inconsequential. Of all her suitors, Phoebus is Esmeralda's top pick. Granted, she doesn't fully realize what a womanizing dick he is, even after a disturbingly rapey scene where he very nearly succeeds in deflowering her against her will. What a catch.
Bachelor #3: Frollo. This dude wins the creeper award on like six different levels. He's a priest, first of all, and has never known the charms of a woman until he sees Esmeralda dance. A cumulative lust of 30-odd years is suddenly awakened, and Frollo finally decides to take her by force - but, his unsuccessful attempts to murder Phoebus and rape Esmeralda only fuel his sexual agony and sense of damnation. A holy man who suddenly believes himself beyond salvation is capable of anything, and Frollo becomes the most dangerous character of all.
Bachelor #4: Quasimodo. Let's face it - Quasi never had a snowball's chance in hell when it came to winning Esmeralda's affections. Like Frollo, he falls for the gypsy after watching her perform in the square outside the cathedral, but his subsequent actions prove him the opposite of his troubled master. When Esmeralda is arrested, Quasi dramatically rescues her and offers sanctuary in his own bedchamber. She's horrified by her unseemly hero, but Quasi never tries anything funny - in fact, he gives her a rape whistle to use if she is in distress, and sleeps outside her door like a protective, loyal dog. Whereas Frollo is blinded by lust, Quasimodo seems able to put Esmeralda ahead of himself: when he learns that she is pining for Phoebus, he even goes on an all-night stakeout to try to intercept the Captain and bring him to her. Now that's selflessness.
You may have guessed that unlike the Disney version, Hugo's tale doesn't have a happy ending. Esmeralda is eventually hanged for witchcraft at Frollo's behest, and in a fit of despair Quasimodo throws Frollo to his death from the tower of Notre Dame. Quasi dies, too of starvation: his warped skeleton is found in a mass grave, clutching the remains of Esmeralda.
To me, the most compelling thing about Hugo's novel was its tangled treatment of lust and love. We expect this conflict when contemplating the male characters: Phoebus and Frollo think only with their dicks, to ill effect; Gringoire hopes briefly for some action, but is harmless when he doesn't get it; and Quasi knows enough about the world to understand that any intimacy is out of the question for him, and turns his attraction into servile devotion. In this five-way love pentagon, Esmeralda is the real wild card.
It's interesting, first of all, that Hugo doesn't paint Esmeralda as more of a seductress. Perceptions of gypsy culture at the time he was writing would certainly have lent credence to her portrayal as a mystical slut. To style her as a wide-eyed virgin who defends her modesty in the presence of lusty men is to give her the same treatment as a standard heroine. I wonder if Hugo chose to depict Esmeralda this way because she is not, in fact, a "real" gypsy: we are told that she was a native Parisian stolen by gypsies and raised among them. Perhaps Hugo is making a point about incorruptibility: that propriety is an inherent trait of civilized (read: white) women, and can't even be undone through years of fraternization with more wanton ethnic groups.
In fact, I have a problem with Esmeralda's sexuality as a motif in Hunchback, overall. One of the ongoing plot threads in the book is Esmeralda's search for her birth mother; it has been foretold they will reunite someday, but only if her purity remains intact. This is one of those classic "lol wut?" 19th-century tropes. Why does virginity or lack thereof make it any more or less likely that a girl would find her long-lost mother? A woman's chastity is often her greatest power in Victorian novels, and I can never get over how lame a secret weapon that is.
Since sex is supposedly out of the picture, we're supposed to expect that Esmeralda's choices in men are based on something much more pure: true love. Nothing could be further from the truth, though, as her heart is set on jocky assclown Phoebus. Of the four men who are interested in her, he presents the least likely case for love. Gringoire and Quasimodo have actual reasons for loving her as a person rather than a piece of ass, since she effectively saved both of them from death (delivering Gringoire from the mercy of the lynch mob, and bringing water to Quasi when he was on the verge of fatal dehydration in the stocks). Even Frollo has a better case; he is willing to give up a heretofore vow of celibacy and prayer and abandon his vocation just to be with her - a rash sacrifice that no other woman has ever tempted him to make. Phoebus, by comparison, can get play anywhere he wants it, and Esmeralda is merely a more exotic conquest than usual.
The message Hugo seems most intent to deliver by the end of Hunchback is that people are rarely stereotypes. Quasimodo is the most sympathetic character, even alongside the innocent Esmeralda -- he may even be more sympathetic, in fact, because we know him to overcome a life of solitude, abuse, physical handicaps, and lovelessness. Frollo illustrates the point that even people who dedicate their lives to education and prayer can fall victim to baser human instincts, and the contrast between Captain Phoebus and the poet-cum-thief Gringoire proves that chivalry and goodness of character has nothing to do with one's social standing.
And as for Esmeralda, well, I'm not sure. Perhaps we're just supposed to be relieved that at the end of a long story driven by the caprices of men, Esmeralda has succeeded in her sole duty of safeguarding her purity. In this narrative arc set against the backdrop of the most famous Catholic church in history, the heathen gypsy girl is in fact the only character qualified for heavenly salvation after her death - everyone else is a murderer, thief, philanderer, or blasphemer. Sweet irony... now there's something Disney never delivers.