I'm not really one for writing long reviews of anime I've watched, but I felt that I had to say something about this one.
Darker than BLACK - Kuro no Keiyakusha had a dark storyline and characters with very distinct traits, especially Hei. Darker than BLACK - Ryuusei no Gemini, the sequel seemed to forget these.
*Mild Spoilers Ahead*
This anime would have been great if it was a stand-alone, but it was unlucky enough to have expectations to live up to its predecessor. The main character, Hei - once a calm, collected, cold-blooded killer now stars as an umkempt drunk who abducts small children and beats them.
Mao, the cat who had previously died, is now a flying squirrel and stays as one even though he had many oppertunities to possess a cat. Yin has turned from a near-silent doll into a... wait, nothing at all was explained about that - but she made a lot of contractors commit suicide so that she could... that wasn't explained either. Kuro no Keiyakusha kept the plot a complete mystery until the end, but Ryuusei no Gemini didn't really explain it at all.
So, our old friends from Kuro no Keiyakusha have had their characters changed drastically, but what about the new faces? Maybe they will make up for this? A loli with a rifle; a pervert who can turn into a golem and a lesbian with a lightsaber... personally, I didn't find them very original to say the least.
Enough of bashing the characters though - onto the story. The series was half the length of Kuro no Keiyakusha, and the two episode long arcs were discarded. I was dissapointed by this since too many things weren't explained, even for Darker than BLACK. The last episode really needed to be two, maybe three, episodes and again explain a few more things. It was a cliffhanger disguised as closure.
Overall, a decent anime. If you do plan on watching it, have Kuro no Keiyakusha ready to watch again so that you can get rid of the bad aftertaste - Ryuusei no Gemini doesn't live up to the original and makes you think less of some once great characters.