Naraku

Onigumo was the human that made up Naraku, he was badly burned (i believe) and u only see him mummified like, but anyways Kikyo finds and takes care of him and he sorta falls in love with her (but its obsessive creepy love) and since he won't recover from his wounds and since he want's power he invites hundreds or thousands of demons to infest his body creating Naraku. One defining feature had something to do with a spider like burn on his back which stays with him even as Naraku and reminds him that he was once human which he of course now hates.

Kagura is a Wind Sorceress she has control of the wind. She kills most of Koga's pack and its her that he wants revenge against.
Kanna appears to be a small ghost like girl, she has a mirror that can suck in a human's soul, then she can use the mirror to manipulate the human's body. She can also project whats happening in current time through the mirror which she does for Naraku (kinda like live tv).
Goshinkishi- Naraku's third creation. Goshinkishi is telepathic and very fast he can read the minds of his opponents so he already knows what they plan to do. He breaks the Tetsaiga with his teeth but that cause Inuyasha to go into full demon mode killing him. His teeth were later used to create Sesshomaru's new sword Toukijin.
Joromaru and Kageromaru- creepy pair Kageromaru is the brain of Joromaru. Kageromaru has a mask over his face when they first meet him once he removes it his parasitic twin comes out of his mouth. Kageromaru looks 'human' enough but Joromaru is a tiny creature with a 'human' face a snake like bit of body and arms with scyths for hands, they are both very fast and Joromaru can go underground and pop up anywhere.

The Thunder Brothers- Hiten and Manten
These two are the ones who killed Shippo's father.

Yura of the Demon Hair
Yura was one of the first demons faced by Inuyasha and Kagome, she could control people with long strands of hair.