Okay, so tropes for Lilia x Reader fics? Honestly, a lot of the usual yandere or dark romance stuff feels off to me because he's canonically so ancient and has a specific, mischievous but ultimately paternal vibe. I think leaning into that weird gap is more interesting. The reader being someone who stubbornly refuses to be coddled or protected, who keeps stumbling into his centuries-long burdens, forces a different dynamic. He can't just be the scary general or the cute bat, he has to deal with this mortal who sees the cracks.
Found family tropes but twisted a bit—he ‘adopts’ the reader, but then the feelings get complicated. Or time travel: the reader is from his past, someone he failed to save, or from the future knowing how his story ends. That plays with his loneliness and his sense of duty. I'd avoid making him overly possessive in a modern boyfriend way; his possessiveness would be quieter, more about strategic manipulation to keep you safe within his sphere, which the reader might resent.
Ending on a lighter note, comedy works too—the reader is just relentlessly normal and unfazed by his antics, treating him like a weird roommate. His dramatic flair meets utter domestic mundanity.