Kagerou Project

チルドレンレコード by Shidu

Kagerou Project, abbreviated as Kagepro, is a series of songs that tell the story of children with red eyes and 'eye abilities', composed by Shizen-no-teki-P (or more commonly known as Jin) and artwork and videos by Shidu and Wanyannpu. The series has been wildly popular on Nico Nico, and has released two albums so far. It also has several novels and a manga, and soon anime, adaptation. 

The story begins with a seemingly normal scenario, disregarding some not-so-normal details such as a AI girl living in our protagonist's electronic devices. Said protagonist, Shintaro, is a hikikomori, who has dropped out of school despite his extremely high IQ, and have never left his house for two years. That is, until he finds his computer broken. He then proceeds to go to the department store and, instead of buying what he needs and peacefully return to his shut-in lifestyle, the department store gets attacked by terrorists, and he becomes one of their hostages. Meanwhile, his younger sister, Momo, a popular idol, encounters Kido, who is apparently the leader of the 'Blindfold Gang', which has just 4 members at that time, all children with eye abilities. Momo discovers that she has the 'attracting eyes' ability, which was partly what enabled her to be so popular. She tags along and eventually encounters her brother, and together with the gang, they use their abilities to save the customers of the department store. Shintaro and his AI, Ene, who also has an ability, are forced to join the gang soon after. Strangely enough, Shintaro does not have an ability like the others. Later on, the story slowly unfolds and shows the reason why Shintaro became a shut-in, the significence of that reason, and eventually, the creator of those abilities. Their abilities are helpful, but they were obtained at a great cost - the life of someone they hold dear. This is shown best in the song Kagerou Days, or Heat-Haze Days. Two children, Hibiya and Hiyori, were forced to loop each other's deaths repeatedly, apparently for more than 10 years, inside the heat-haze. It only ends when one of them breaks out of the daze, while the fate of the other is unknown. Hibiya broke out and obtained an ability, but Hiyori's fate remains unclear. 

It is highly speculated that the one left in the haze dies, or simply joins the creator of the haze in it forever. The series gets darker towards the end, and eventually the story seems to get a 'bad end' where everyone dies. But then the timeline is reset, and they apparently obtained a 'good end' eventually.