Asanagi was once a regular human who survived the invasion of Earth by the interdimensional demons referred to as The Unknown. In the ruins of Kyoto, he was the last human survivor, and was hiding in an alleyway from The Unknown in order to survive. However, while there, he was captivated by the beauty of Airi Yunami, a leading member of the Unknown who was possessing a human body.
As he ran to her, one of her subordinates stabbed him in the stomach with its hand, and he awoke hours later to discover that Airi was having him experimentally modified into a human-unknown hybrid with superhuman strength, durability, and the ability to manipulate winds. Asanagi became utterly devoted to Airi and devoted his life towards the goal of a future with her, and to his surprise she developed similar feelings towards him.
The humans in the various invaded Japanese cities had placed their children in "cold-sleep" in order to protect them from the Unknown during the invasion, but after the humans abandoned Japan Airi and Asanagi began eperimenting on the humans and giving them superhuman abilities referred to as "world", in order to make them more like the Unknown (this was Airi's substitute for having children, since true Unknown are unable to reproduce, she was spreading her genetics into humanity).
After the Unknown were defeated by much of the world's military, Asanagi and Airi began awakening the children and brainwashing them with a holographic version of the world that made all Unknown look human, projected into the heads of the children by hips impanted into their heads. This fake world also made real humans without chips look like Unknown.
As the fake leader of the Japanese Defence Force, Asanagi woke the children and had them split up into schools who would fight against the Unknown (actually humans) using their various abilities, motivating them using a "points-ranking system". For most of the series Asanagi and Airi appeared as supporting characters helping, advising, and ordering the series' protagonists supposedly for the good of humanity.
After Suzaku and the rest of the main characters were rescued from The Unknown by the real Japanese navy (lead by Asuha and Kasumi Chigusa's mother), the various children with The World launched an assault against The Unknown. Airi ordered Asanagi to defend the portal against the children, when he was confronted by Suzaku, Canaria, Kasumi, Asuha, Maihime, and Hotaru.
At first presenting himself as a regular human, Asanagi seemed utterly outmatched, until Aoi suddenly betrayed the rest of the group, believing that the love Airi and Asanagi had shown her over the years was real, and used her world to take away the powers of all of her comrades, before fleeing to be pursued by Asuha, who was forced to strangle her to the point of unconsciousness, before leaving her for dead. Asanagi then unleashed his true power and battled intensely against Suzaku and Kasumi, until he realised that Airi had been killed by Maihime and that the Unknown's portal had been destroyed. Since the portal's evenrgy was the only thing keeping the compound together, Asanagi simply accepted defeat and told Suzaku and Kasumi to leave, seeing there to be nothing left for him worth fighting for.
Asanagi accepted death and was apparently killed by the exploding compound only minutes after lettng Suzaku and Kasumi go. However, it was later revealed that he had survived, and was simply falling through the compound, where after landing he encountered the barely alive and unconscious Aoi.
Asanagi was then met by a ghostly image of Airi, who told him not to leave Aoi to die along with both of her "parents" (meaning herself and Asanagi), and told him to take care of her. Asanagi picked up Aoi from the floor and left, vowing to look after her as the father he should have been.