He is a small, doll-like Shinigami whose appearance suggests that he was poorly sewn together from mismatched fabric. He only has one eye, despite having two eye sockets.
Gelus died because he'd fallen in love with Misa Amane. On Misa's final day, he and another shinigami, Rem, watched what'd happen. Before Misa's destined murderer can kill her, Gelus uses his Death Note to kill him, against Rem's protests. He drops his pen and start crying, as his eye disappears and he disintegrates. Gelus extended Misa's lifespan, and was reduced to a pile of dust as punishment, leaving behind only his Death Note. Rem then delivered his Death Note to Misa because it was her that he saved, as she thinks that was his final wish.