If you're looking for anime similar to Darker than Black: Kuro no Keiyakusha Gaiden, you might like these titles.
For Kouta and Yuka, finding the bloody naked young girl on the beach would change their lives forever, for better or for worse. Unable to speak or function as a normal human being, she is named Nyu by the duo, and taken into their home in an effort to save her. But what neither teenager knows is that this innocent young girl is actually a killing machine -- an experiment gone terribly wrong -- and it is only a matter of time before the murderer in her awakens again...
Both these series hinge upon:
- using "monsters" to show how fragile and important positive emotions are in making the world worth living in...
- very logical characters...
- getting very "dark" in content when it moves the emotional story forward.
The differences between the two are only that Elfen moves slower (it's 3 years older so it's expected) and has even fewer "laughs."
Kijima Naito and Fei Zhui Lau are fugitives of a laboratory that cruelly researches the effects of the Sacred Seven crystals on humans. Parallel to what is happening in the Sacred Seven TV series, this is the story being told from their perspective.
Following a violent incident in his childhood, Alma Tandoji has been left an outcast at school and wants nothing more than to be quietly left alone. But one day Alma's solitary peace is shattered when a strange monster known as a Dark Stone attacks his hometown. Suddenly the boy finds himself thrust into the struggle between this strange creature and the ones fighting it: the Aiba Foundation and it's leader, the wealthy Ruri. Though reluctant to fight at first, amidst the battle, Alma learns that he possesses a power known as Sacred Seven that is linked to not only these mysterious creatures but also his forgotten past. Now, along with the help of Ruri and her bestpectacled butler Makoto, Alma must take up the mantle of protector to save the world from this new threat.
Some years into the future, a strange, unexplainable natural disaster has broken off a piece of the Japanese mainland. Known as the Lost Ground, it is now inhabited by people with Alters -- machine-like extensions of their own will. In order to keep the peace on the Lost Ground, the HOLY organization uses its own Alter capabilists to police the region. In the midst of the chaos is a young man named Kazuma, whose life will soon change forever as he discovers his hidden abilities, and how to wield them.
Several years after the incident in Amazawa town, Chinami Ebihara is now a member of an organization - founded by Sonomi Kujo - called OZ. Like its rival Foundation, OZ studies those with the special power known as 'Type-E' - the ability to affect electrical devices. However, while OZ wishes to help Type-E users gain control of their powers and be productive members of society, Foundation wishes to gain their power through force and use them for unknown purposes. Along with a fellow Type-E user named Maori, Chinami goes on missions to determine Foundation's motives and protect other Type-E users from their grip.
Gawl, Koji and Ryo may seem like ordinary exchange students, but in reality they're anything but normal. The trio has travelled into the past to stop dangerous genetic research from being completed – research that has terrible consequences in the future – and Gawl, a boy with the ability to morph into a powerful creature, may be their best hope in succeeding. Now, between trying to hide their secret from the nosy daughter of their landlord and attending classes at Oju Academy, the boys must race against the clock to complete their mission at any cost.
In the near future, the poor dwell in a place called Night Town, where survival is the order of the day. While working for a delivery service, a boy named Hajiki stumbles upon a strange cube called a Gad, which turns into an iron man when his life is on the line. All he ever wanted was to live a decent life with his family, but now he must fight against others to defend all that he holds dear.