"A broad-backed man, baldish, in an oatmeal coat and dark-brown trousers" (2.18.3). Clare Quilty is Humbert's "brother" in lust, his double and worthy opponent in verbal skills. Quilty is a playwright and an admirer and maker of child pornography. Indeed, his love of young girls has already once run him into trouble with the law. By his characterization, Humbert manages to make Quilty seem even more corrupt and more perverse than him. To Humbert, Lolita must be protected from Quilty. Where Humbert "loves" and idolizes Lolita, we find out later that Quilty wants her to star in his porn movies, which is why he takes her to the Duk Duk Ranch.
Quilty fervently trails Humbert and Lolita across the United States, leaving behind a trail of provocative clues (literary references, initials, inside jokes) in motel ledgers, generally making Humbert into a (more) paranoid and jealous lunatic. Switching cars and lurking around every corner, Quilty finally "abducts" Lolita from the hospital in Elphinstone – though she is all too willing to go and has been in cahoots with him all along. Remember – she planned the itinerary for the second trip.
Even facing death, Quilty takes nothing seriously. As Humbert aims a pistol at him, he declares, "Now, soyons raissonnables. You will only wound me hideously and then rot in jail while I recuperate in a tropical setting" (2.35.77). He proves tough to kill, but after many shots, Humbert takes him down, reducing him to a "purple heap" (2.35.86). Shockingly, he is the only man Lolita ever really loves, or is "crazy about."