I've been looking for this book I read a few years back. I read it online and I have no recollection of the possible cover page. It was definitely fiction, romance (adult themes), unclear on whether it was considered fantasy or not.
From what I remember, the plot is about the FMC, who is the daughter of an evil king who I think ran away from the castle (as I'm writing this I'm starting to doubt if she was kidnapped but I'm 90% sure she ran away and was in hiding) that stumbles across the notorious rebel force. They take her (willingly? unwillingly? I'm uncelar on that one) with them to their base, which is underground. She then starts training with them and has a weird flirty but antagonising relationship with the MMC, who is the leader's son. He's the snarky grumpy type I think, name possibly something like Darkus? Darius? Something D-shaped (hah) likely. They lived in dorms and I think there was another guy that was hitting on her - one scene I remember is them hanging out and MMC guy leaves? Never shows up? to be all dark and gloomy and she goes to find him and I think adult stuff happens? i distinctly remember that for a romance book there were too few adult scenes, likely only two. The book I think was the first of a series? It ends with them discovering she is the daughter of the king who killed MMC's mother (which is what led to his father starting/joining the rebellion? and the guy is very hellbent on killing the king) and her running away. I think the final scene is set in a forest, I don't remember if they're back on the surface for a reason or if she's escaped - maybe someone warned her and let her leave? Memory's really fuzzy.
Every time I try to google it a bunch of other books show up because the concept is very classic romance trope so I hope you guys can help! Thank you in advance, mwah mwah!