Dark is another personality that lives in Kano's mind. He lives in the Morgue and "guards the doors so that nothing bad gets in." Kid calls him his brother and seems to camp in the Warehouse near the door to the Morgue, suggesting that they may be close. However, Kid is scared of his anger which manifests as his shadow, the Breaking Noise. Dark seems somewhat possessive / protective of Kid and Nadia and pretty much hates Fuuka at first contact.
When Nadia is awakened by Fuuka's attempts to break through the Morgue, he manifests the wings of fire that Kano did when contacting the Eye. Dark has at least once taken over Kano's body on Earth, to defend him against assault by Josh, a homophobic fellow inmate of the psych ward.
Dark looks like a taller, paler, sickly version of Kano, with short hair and covered in scars. He seems to carry a lot of physical wounds: a nervous habit of clawing at his face has left it covered in criss-cross scars, his arms are bandaged, there's a sewed-up gash across his throat, and his mouth is stitched shut. His eyes are red, though he recently gouged them out to use the blood to write on Red's cage while they were talking. Recently he has inherited Red's trademark facial scar, which Red claims belonged to Dark in the first place.
When Dark manifested on Earth, he did not yet have all the scars and stitches that he does now. His wings were black and red at that time, but the feathers were falling out, heavily implied to be another symptom of Red's introduction into Kano's mind.