Dying Waves

Of all the creatures that Sam and Dean had faced, not once had they thought that such beings as mermaids were real. That is, until they discover her; tied up, bleeding and slowly dying. Against their better instincts, they brought her along with them, nursed her back to health and taught her the ways of humans. The one thing she didn't expect was to gain a tidal wave of emotions.

(A Dean Winchester/OC fanfiction starting from season 2.)

Nerissa (The OC) is portrayed by Phoebe Tonkin.

Nerissa means From the Sea or Sea Nymph/Sea Sprite, it's Greek.