Actually, I've been hunting for this exact kind of character for a while, and it's way more niche than you'd think. A lot of stories with concubine-like dynamics tend to center on women, but the male version flips some fascinating power and social scripts. The closest I've found is in Chinese danmei novels, where the 'male concubine' archetype often appears in harem settings, but 'concubine' as a direct, primary identity for the male lead is surprisingly rare.
One that comes close is the novel 'The Wife is First'—it's a rebirth story where the male lead was essentially a political tool, a 'wife' in a male-marriage arrangement who had to navigate a complex hierarchy not unlike a concubine's position. His conflict comes from balancing survival, reclaiming agency, and dealing with his complicated feelings toward the powerful male 'husband' figure. The emotional core is less about romance and more about strategic vulnerability and finding strength within a constrained role. It’ drops the flashy power fantasies for a quieter, more psychological tension.
For a slightly different take, some historical BL manhwa like 'Painter of the Night' feature protagonists who are coerced into a concubine-like servitude to a powerful, often cruel noble. The conflict is brutal and revolves around submission, survival, and a twisted form of dependency. It’s definitely not a light read, but the character’s internal struggle between resentment and a desperate need for security is the whole driving force of the plot. These stories treat the role with a stark realism that’s hard to forget.