In terms of Creed, what is the agreed upon essential nature of Satan?

All perfect praise be to Allaah, the Lord of the worlds. I testify that there is none worthy of worship except Allaah, and that Muhammad, sallallaahu ‘alayhi wa sallam, is His slave and messenger.

Satan was created from fire. On the authority of ‘Aa’ishah, the Prophet, sallallaahu ‘alayhi wa sallam, stated that Jinn were created from fire. [Muslim] Allaah Says (what means), {He (Satan) said, "I am better than him. You created me from fire and created him (i.e. Aadam) from clay."} [Quran 38:76]

Imaam Al-Hasan Al-Basri mentioned that Satan was a jinni, not an angel. [At-Tabarrani]

Allaah Knows best.