esm3rald
Blair Gilbert is Elena's older sister and a witch-a siphoner, really, but Damon Salvatore doesn't know that. Damon needs her help with opening the tomb and so they strike a deal: he stops killing locals, fakes being her boyfriend, teaches her all he knows about vampires, and she helps him get what he wants. Simple.
Except nothing is simple when it comes to the Salvatore brothers. Stefan can't stop wanting the wrong Gilbert sister after kissing Blair at the back-to-school party like he wanted to devour her, even as he tried to compel her to forget before running back to Elena. And Damon is far better company than a murderous immortal should be (which complicates the 'no feelings involved' clause).
Blair soon realizes that holding on to control-over her magic, her body, her emotions-might be close to impossible, especially as she's running out of reasons to keep Elena in the dark.