So, urban fantasy has seen the rise of the vampire boyfriend (Angel of Buffy fame). It has seen some werewolf action (Jacob Black in Twilight). The fairy boyfriend? Well, he'll want you to spell it faerie, but Rath Roiben Rye (from Holly Black's Tithe) has got you covered. Gods of ancient civilizations (Hades in The Goddess Test) are coming out of their temples to accept mortal love. Even demon boyfriends are rolling into the romance business (from Anne Bishop's titular Sebastian to the monotone Hnikarr in The Demon's Lexicon), along with their demon-hunting opposites. But publishers are always going to be on the lookout for something fresh and new. Sooner or later, they're going to need a new creature. Someone darker and edgier.
Someone like Cthulhu.
Many complaints have been put forth at the idea of a love story with a Lovecraftian monster. Overall, I've found they boil into three primary objections to the character:
(1) It is a being of immeasurable malevolence: feared intrinsically at the level of the human subconscious, the subject of worship to cults across the globe, and operating on a cosmic scale by which human works are ultimately insignificant.
(2) It is trapped at the bottom of the ocean.
(3) It has tentacles for a face.
Points 2 and 3 are hard to view as anything but an attempt to dodge addressing the proposal in any serious capacity. 2 is merely a matter of location--it isn't an obstacle so much as the opening for a plot device. True love has survived a long drop into the sea before. (SEE: Titanic.) By comparison, 3 is even more easily dealt with. If a love interest can't grow to look past a little thing like physical appearance--well, there's always manifestation via an attractive human vessel. This has its precedents in urban fantasy as well. ("The Demon's Surrender" and "The Devil Inside", though by different authors, are both popular urban fantasy novels featuring love interests who have taken spiritual possession of bodies not their own and successfully engage in emotional connections nonetheless.) It even presents the opportunity to discuss another reigning theme in urban fantasy as well as paranormal romance--one's presentation versus one's true nature.
Of course, there is the natural concern that, by entangling Cthulhu in a romance, one would go against the central theme in Lovecraft's work, which is generally interpreted as one of cosmic indifference to humanity and encompassing a mystery beyond mortal knowledge. But this in itself isn't far from a central theme in urban fantasy: that the world is greater and its dangers sharper than mundane life might suggest. The idea that eldritch beings are waging war beyond human sensing would hardly be a new theme in the genre. Slotting in an existentialist philosophy to fit this perspective seems even less out of the question.
Humanising Cthulhu is understandably more difficult to swallow, particularly for older fans. Much of the impact of its character, after all, exists in its ability to provoke fear. It's worth noting, however, that urban fantasy has a long-term ongoing cycle for the reclaimation and redemption of monsters, ranging--as seen in the introductory lists--from half-human monsters to divine ones. With darkness gradually coming to steep the genre--courtesy in part of mainstream publishing's constant desire to up the stakes--Ctlhulhu as a significant figure with a built-in fanbase must stand very much in line as a possibility for consideration regardless of its origins. Its nature is scarcely any more a deterrent--much of its malevolence reflects on its incomprehensibility rather than directed malice. Unlike vampires, Cthulhu doesn't possess the tension of regarding humans as actual prey. Frankly, it comes down to a very simple logical process. If it has desires and intellect, it has a mind. If it has a mind, it has a personality. If it has a personality, it may be understood--and loved, however slender its chance. Isn't that gamble the pinnacle of romance?
Let nothing stand in the way of true love. It's what the genre conventions would want, after all.