But still it certainly ridiculous to think about Yin as a real love interest. She's more like a new sister figure for Hei.
Though she may think about him otherwise...
Same with Misaki somehow. She had with Hei(and Li-kun) a certainly interesting development. If Li-kun were his original face he would've came to love her, but it was only a facade of his. And he knew well how dangerous and impractical to start a relationship with someone who actually chases him(BK-201) most of the time.
And most of all I can't consider Yin as a real love interest because even if she had emotions it's not much and she had no other qualities other than her ability. She's a totally one-dimensional character who barely speaks. Her arc was nice though...