Spiral: Suiri no Kizuna (スパイラル～推理の絆) is a mystery manga which was later turned into an anime by TV Tokyo.

Spiral: Suiri no Kizuna is a shonen manga by Kyou Shirodaira (writer) and Eita Mizuno (art). It was published in Monthly Shonen Gangan, animated into a 25 episode series, and dubbed into English by FUNimation. Both the anime and manga are completed, the manga lasting fifteen volumes. The anime is at best an incomplete work in that it ceased production before any of the mysteries of the story were resolved. The anime has also debuted on American TV in a syndicated FUNimation Channel programming block airing on CoLours TV beginning on Monday, June 19, 2006.

Spiral is a mystery story with a plot that is much deeper than other detective series, like Case Closed (Detective Conan). The driving mystery of the story is the nature of the Blade Children, those who have cat-like eyes and are missing a seventh rib bone.

The Spiral manga was licensed by TokyoPop, but was later dropped. A new manga publisher, Yen Press, has picked up Spiral and will be releasing the first volume later this year.

Ayumu Narumi's older brother Kiyotaka, a famous detective and pianist, disappears without a trace. The only clue as to his whereabouts is the "Blade Children", the only words Ayumu managed to catch in a final phone call from Kiyotaka. Two years later, a row of murders and incidents begin, all related to the Blade Children. Together with the infamous (and only) school journalist, Hiyono Yuizaki, Ayumu tries to figure out their destiny and the mystery shrouding their past.

The Blade children are the mystery of the series/manga. They are cursed children that few know about and are being pursued by so-called Hunters. Of the main features of the Blade Children are their glowing eyes with their slit-shaped pupil (similar to cats and snakes). They are also missing a seventh rib bone on the right of the rib cage. All their fathers are unknown.

The story presented in the anime never reaches a point where the nature of the Blade Children is explained. The anime only offers a comparison to Cuckoo birds. The implication being that something not human deposited the Blade Children in human "nests" to be raised, similar to how Cuckoo birds deposit their eggs in the nests of other birds. The anime also suggests that Cuckoos go destructively and violently crazy toward the end of their lives.

The manga explains why the Blade Children are cursed and their origins:
A man called Yaiba Mizushiro was born sixteen years before Narumi's older brother, Kiyotaka. He was born with one rib missing from his right ribcage. Like Kiyotaka, he excelled at everything he chose to put his mind to. Like Kiyotaka, he was the darling of the world for the first twenty-odd years of his life. When he turned twenty-three, Yaiba started his own secret society, which swiftly grew to gigantic proportions, able to manipulate world events. Citing boredom, Yaiba initiated the "Blade Children Project".

Using in vitro techniques, he seeded his DNA and created a total of eighty children. These children all had a rib removed at birth to signify them being of Yaiba's blood. They were cursed in the same way Yaiba was: they would grow up as geniuses in their own right, but one day their blood would awaken murderously and take over their self-will, becoming avatars of Yaiba.

Yaiba's organization was split into three camps over the Blade Children Project:

• Savers. The first group, Savers, were supportive of Yaiba and desired to create more Blade Children.
• Watchers. The second group, Watchers, were neutral. They wanted to observe the first batch of Children and gather results first.
• Hunters. The last camp were called the Hunters. They were against Yaiba, and tried multiple times to assassinate him. Yaiba had the devil's luck, and everything failed, from bullets to bombs.

When Yaiba was thirty-six, one Japanese man came out of nowhere and easily killed him: Kiyotaka Narumi. He was the counterpart to Yaiba: if Yaiba had been a destroyer, Kiyotaka was a creator. Yaiba had intended to remake the world literally in his own image; with his death, the Blade Children project was halted. Kiyotaka had his hands full trying to stop the Hunters from killing all the Children, while trying to check the Savers at the same time.

The counterpart metaphor does not end there. If Kiyotaka had a brother who was like him but not him, another creator, then Yaiba must also have a sibling. That is Hizumi Mizushiro, Ayumu's mirror image, the one who will awaken the blood of the remaining Blade Children.

- From Wikipedia