It depends on what the mangaka want the demon to look like.. ^^
For example.. There's Rin Okumura from Ao no Exorcist. <3 He has this sword that makes his true powers come out. (Hard to explain) He's the son of satin, but hides it.. ^^
Kenshin's battousai manslayer form is quiet demon-ish. He is the main character of Rurouni Kenshin. And his power is killing. But, Kenshin has turned a new leaf and has decided never to kill again, though temptation and pressure from other manslayers often disrupts his transformation process. Kenshin won't allow anyone to hurt his new family: Kaoru, Sanousuke, of Yahiko
A shinigami this one certainly looks kind of like a human, a shinigami's power is to kill a human being door useing it's Death Note, also a shinigami has wings.