According to Haumea, at the time Shinra Kusakabe defeated Leonard Burns, Iris became the Eighth Pillar, during which she used Adolla Grace

Adolla Grace

The Adolla Grace (伝導者の加護, Dendōsha no Kago) is when a Third Generation's pyrokinetic abilities are enhanced by an Adolla Link with an otherworldly being, such as the Evangelist, making the pyrokinetic a Fourth Generation.

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to shield Special Fire Force Company 8 from Dragon's Ignition Ability.

How is Iris the 8th pillar?

Is Iris the Eighth Pillar? Iris is a sister originating from The Holy Sol Temple, and a member of the Special Fire Force Company 8. Unbeknown to her, Iris is the 8th Pillar thanks to the schemes of Sister Sumire. Iris is the Doppelganger of “Amaterasu,” i.e., the First Pillar.

Which pillar is the strongest Fire Force?

Benimaru Shinmon

At our top spot is the strongest Captain in the Special Fire Force, Benimaru Shinmon (or as he likes to be called: Shinmon Benimaru). What makes him stand out from the other Captains? He's a Second and Third Generation Pyrokinetic, which grants him a pretty destructive skill set.