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As an Infernal, Saeko retained most of her feminine features; having a lean figure and flowing hair that were formed of flames. She had black charcoal-like skin, with parts of her body enveloped in flames. She has no mouth and solid-coloured eyes with black sclera.
As an Infernal, Saeko showed peculiar intelligence. As usual to Infernals, she was driven by rampage and chaos, and burned everything in sight. However, she was protective of the Kawaguchi Metal Fittings Corporation, desperately trying to get the Fire Force out of it when they confronted her. This could be due to her husband's affiliation with the building and having a personal affection towards it.
Being a First Generation, Saeko had a set of fire-related abilities, such as fireblasts and fire-enhanced melee combat. Agile and fast whilst in battle, Saeko was mentioned to be faster and more hostile than the regular Infernal.
Upon turning into a Infernal, Saeko goes berserk in her husband's workplace, forcing the Special Fire Force Company 8 to arrive at the scene, in order to rescue the employees inside. Sneaking up behind Shinra Kusakabe, she absorbs the flames around her and strikes the boy, blowing him away and leaving him unconscious. As Shinra regains awareness, Saeko rushes towards the brigade. She is stopped by Akitaru Ōbi, with Maki Oze blasting away the flames to prevent her team-mate from getting burnt. The man then uses his weapon, aiming to impale the being's core, but Saeko hits the weapon out of the man's hands, forcing the brigade to separate to safety. As Shinra recalls his past and the promise to his mother, he charges towards Saeko with a blazing kick, successfully impaling her core, thus sending the woman's soul away into the raging flames.