Fire Force season 3 episode 12 shockingly ended the first cour, with Shinra Kusakabe seeing a glimpse of the world before the First Cataclysm. The world appeared fundamentally different, including the humans inhabiting it. It was as if Shinra was seeing a live-action footage of our world. The strange visions flooded his eyes as he tried to suppress himself from plunging into an ancient madness.
The next moment, Shinra woke up and saw himself chained to a bed. A live news report on the television beside him confirmed that he had been unconscious for three months. Interestingly, Shinra Kusakabe appeared slightly different, with his hair dyed blonde. Undoubtedly, anime-only viewers may want to know more about the perplexing cliffhanger, including Shinra's change in appearance.
So, why did Shinra's appearance change at the end of Fire Force season 3 episode 12? According to Atsushi Ohkubo's manga, Shinra Kusakabe was possessed by his Doppelganger while he was unconscious.
Shinra's Doppelganger was a violent criminal, a mirror image of how the world perceived him - a devil. As such, he dyed his hair blonde and did many heinous deeds while Shinra was unconscious. Eventually, his actions forced Shinra's friends to detain him.
Disclaimer: This article contains spoilers from the Fire Force manga.
Exploring Shinra's change in appearance after he glimpsed the past in Fire Force season 3 episode 12

As mentioned, Shinra Kusakabe appeared different at the end of Fire Force season 3 episode 12 because of his Doppelganger. After witnessing the world before the first Cataclysm, Shinra was overcome with fear and madness. The perplexing sight made him stay unconscious for three whole months.
During this period, Shinra Kusakabe's Doppelganger took over his body. In other words, Shinra's Doppelganger lived the boy's life and worked as a member of Company 8. However, the Doppelganger, as seen in Fire Force season 3 episode 12, wasn't "heroic" like Shinra. Instead, he did things that Shinra would never do.

Firstly, he dyed his hair blonde and wore it in an upward flame-like fashion. Shinra's Doppelganger also got his ears pierced, wore a pair of earrings, and got a Devil tattoo. What's more, the Doppelganger slapped Iris, thinking she was annoying. That eventually led the others to detain him.
However, he disappeared when Shinra regained his consciousness. Arthur was the first one to realize that Shinra was possessed by his Doppelganger. Likewise, he could easily observe when the boy had returned to his original senses.

Interestingly, Shinra's Doppelganger, seen in Fire Force season 3 episode 12, was a mirror reflection of society's perception. Since people believed Shinra was evil, cruel, and the Devil incarnate, the Doppelganger resorted to villainous acts.
Moreover, the Devil tattoo was also the result of a rumour, where people believed Shinra had gotten flame tattoos on his ankles to celebrate the death of his family. Interestingly, Shinra's Doppelganger didn't appear like Leonard Burns's version or the others. In other words, the manga doesn't show the Doppelganger having a body of his own.
Conclusion
Fire Force season 3 episode 12 ended the third installment's first cour with a fine cliffhanger, leaving fans guessing about the narrative.
The episode finally revealed how the world looked before the first Great Cataclysm, and showed Shinra Kusakabe in a new look. Interestingly, the change in appearance was due to the boy's own cliffhanger, which had possessed him while he remained unconscious.
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