Spare is a loaded little noun. It means the one who was never the plan, the second name on a list, and pairing it with unlikely sets up a story about somebody stepping into a role nobody intended for them.
Jax Calder writes The Unlikely Spare into a sequence our catalogue lists as Unlikely Dilemmas, and the shelving splits three ways: romance, LGBTQ+, and crime and thriller. Blends of that kind are a deliberate craft rather than an accident of tagging. The suspense supplies the pressure, the romance supplies the reason to care, and neither half works properly without the other.
That is the extent of what the record supports, and it is quite enough to tell you whether this is your sort of book.