Iris

Iris is a sister that works with the 8th Company. She does not have powers.

During her childhood, Iris attended The Holy Sol Temple alongside Hibana, where they prayed for most the days and worship the Sun God. When the church caught fire, she and Hibana looked on as all the sisters were incinerated, leaving the pair as the only sisters to survive the incident.

Later in the manga, it's revealed that Iris is the doppelganger of "Amaterasu" and is the 8th and final pillar, although she isn't aware of this.

(Source: Fire Force Wikia)