Shin Megami Tensei: Devil Children Book of Light/Book of Dark is a role-playing video game from the Megami Tensei series. The US versions are titled DemiKids: Light Version/Dark Version. Similar to its predecessor the two versions each feature a different main character, Jin for the Book of Light and Akira for the Book of Dark. Both main characters are Devil Children - half Demon, half human.
The year is 200X. Jin, Akira and Lena are three child hood friends who like mysterious things. One day in the library, along with the mysterious transfer student Amy, they find the "Akuma Compendium". They chanted an incantation in the book and, to their surprise, devils appeared. Ami then told Jin and Akira that they are the Devil Children that will decide the fate of the world. She hands them their Devil Risers. The group then pass through the "Door of Time" to the land of Valhalla where they fight an evil ruler known as Imperius who plans on conquering all of the world. The Nakama of Jin is Rand, a Sol Lion, and Akira's is Gale, a Hylon.
The gamplay is mostly similar to its predecessor but with newly added elements. Both the Dark and Light versions tell the same story, but told from the different perspectives of the two characters. Certain Demons appear only in the Dark version or in the Light version. Gameplay in the Dark version is substantially different than the Light version; Dark is more complex while Light was simplified.
In America these games suffered due to what fans consider major gameplay flaws. Such as, the Demons don't level, the main Demon companion only levels when other demons are fused/sacrificed to it, and an overly complex menu system.
Source: Wikipedia, "DemiKids: Light Version", available under the CC-BY-SA License.