Simmons was a demon loyal to Lucifer.
Background
Simmons was among the demons summoned to talk with Lucifer after he usurped the throne from Crowley. Simmons suggested the demons deploy a force to locate Amara. She referred to Crowley as a cowardly leader, for which Crowley reprimanded her.
Later on, Simmons helped Crowley escape his cell and followed him to his lockup, where Crowley had hidden the Rod of Aaron. Lucifer arrived shortly after and confirmed that Simmons was only pretending to side with Crowley so they could strip him of his last sense of defiance and maybe locate his own Hand of God, which Lucifer wanted.
Lucifer added that all demons - not just Simmons - hated Crowley, which Simmons confirmed.
When Crowley attempted to use the power of the Rod against Lucifer, Simmons jumped between him and the blast, sacrificing herself. She was disintegrated immediately upon impact.