Insomnia is a mirror artwork created by the Hikari.
Insomnia forcefully brings Daisuke into his world, a reflection of Azumano, and attempts to switch place with him. Under the name "Rokuto Azumi", he makes Daisuke believe they are best friends and copies his appearance, role and even love interest, Riku, to overpower Daisuke and take his place into the real world.
In order to mirror Daisuke completely, he creates a double of Dark, Mute.
Mute appears in volume 14 chapter 2. He is Dark's reflection in the mirror that is Insomnia . He, along with Insomnia, wants to take their originals' place and go to Daisuke's world.