Basically I came up with my main characters after a spate of activity and posts I witnessed surrounding monster x human relationships. A creator I follow was talking about how Ancient Magus Bride was problematic and after hearing other people's issues with it, I'd agree (even though I'd never read it or watched it myself). The creator was making their own monster x human story at the time and I also saw a scattering of related posts on various sites. It got me to wanting to make my own monster x human couple but with my signature spin on it.
1st spin: The human in my case isn't some fair virginal maiden. Well, yes, she's a virgin at the start of the story, but I'm talking about more of a virginal mindset as portrayed in mainstream media. Like the pure, innocent, removed from her own sexuality kind. For my protagonist, I wanted her to enjoy sex and not be ashamed of it or shamed for it by any rhetoric in the narration or by her close friends.
2nd spin: Power balance. Since this is one of the main talking points about Ancient Magus Bride, I wanted an equal "need" between the human and the monster. The monster essentially needs the human as his host to be alive, and she needs him for the company. Here I was largely inspired by the relationship between Eddie Brock and the Venom symbiote (of which also inspired some of my monster's ability).
3rd spin: No gentleman monster here! Unlike a lot of the more civilized monsters, Beast (Beauty and the Beast), Teacher (Girl from the Other Side), Elias (Ancient Magus Bride), and various others from pros and indie creators alike, I wanted mine to be a bit more … feral? Like he can be nice and gentle if he wants, but he can be super possessive, aggressive, and act like a petulant child at times. He'd rather tear his lover's clothes off in an alley than draw a bubble bath surrounded by lit candles for them let's say.
But with those core elements outlined, I basically came up with a story around how my monster and human meet, and how they reach a point where they trust and love each other.