Armonia's appearance is that of a golden skeleton adorned with all manner of jewelry. He was always seen sitting in a special heavily decorated throne made of skulls.
As described in Death Note 13: How to Read, Armonia is the right-hand man of the Shinigami King, and he sits on a throne. Highly intelligent, he knows everything there is to know about the Death Note. Takeshi Obata also describes him as being "conceited."
Armonia provides Sidoh with several scrolls describing the various rules that Shinigami have for interacting with humans, which Sidoh uses to guide his interactions with Mello in the Human World. The scrolls are not mentioned in the anime.
Obata said that for Armonia's body he referenced Tibetan art that features skulls. He also used ideas stemming from Italian antique accessories that feature faces covered in jewels.