A assassin that was sent to kill Rito by one of Lala's arranged suitors. Eventually she found out she was used and broke the contract with her employer. She said she wasn't finished with Rito but is really using this as a excuse to stay on earth.
Golden Darkness is alien weapon created by the scientist Professor Tearju for some orginization. Yami has the ability to shapeshift her body into anything due to the nanamachines in her body. She was freed from her creators by the Assassin named Black and decided to become a assassin as well.
Golden Darkness is assassin from space that was sent to earth to kill Rito Yuki by Lacospo. She first encountered him while he was on a shopping trip. This is when he notices fnotices Yami and is thinking she is giving him the eye. Thinking that she was shy girl that wanted a piece of taiyaki Rito offers the bag. She is surprised, when Yami takes a piece. Once she finishes she reveals her intentions to him and abruptly transforms her entire arm into a blade and almost takes Rito's head off.
Rito then leads her on a merry chase through the city, barely getting out of the way of maces, blades, and flying fists. Along the way the principal of the school happens to catch a glimps of her panties, she beats the tar out of him. Once they get a certain distance from the starting point, Lala comes to Rito's rescue along with Zastin. Zastin fights Golden Darkness (which she was just introduced as) and one of two things happen
Anime: Zastin falls down a manhole into the sewer
Manga: Zastin gets hit by a train.
Lala then steps in and fights Golden Darkness, using her gadgets and flight makes it just about a draw. Rito is inadvertantly hit with friendly fire. It is at this point that the contractor, Lacsopo, arrives and berates Golden Darkness for not getting the job done yet. Golden Darkness drops a bomb of Lacsopo by telling him that she was given inaccurate data. The tiny frog-like man flips out and summons a giant frog, the mucus of which melts only clothing (only in a manga). This same mucus also makes it immune to Golden Darkness's blades and it eventually starts to land small hits (if you can call them that) on her clothing, when the frog is about to land a shot that would have melted everything left Lala jumps in and takes the hit. Again, one of two things happen:
Anime: the frog starts to close on Lala and Rito jumps up and kicks Lascopo off.
Manga: Lala punches the s*** out of both frog and rider after Peke fixes her clothing.
Either way, Golden Darkness decides to delay killing Rito. Even though she never makes another attempt, only bashing him in the face with a fist if he happens to grope her or see her in an awkward moment, like when she uses her "trans" ability to make her chest much larger. She actually comments while looking at herself in a mirror, "I wonder if this will upset my balence..." Rito was passing by and heard this, peeks around the corner... Golden Darkness flips and punches him.
A quaint moment occurs late in the manga, Celine, a flower child that gets drunk and makes people fall in love with Rito, works the "pollen power" on Golden Darkness. So, for a half-hour, Golden Darkness is in love with Rito and even attempts to kiss him. Rito declines, saying that she hadn't been in her right mind. Golden Darkness then completely surprises Rito by smiling.
Golden Darkness lives with Mikado and is best friends with Rito's sister, Mikan.