Gyomei Himejima/Synopsis

History
Before Gyomei was a Demon Slayer and Hashira, he lived with nine other children who were orphans and raised them in a temple. The area was well known for demon attacks so Gyomei had always burned Wisteria Incense to ward off any demons that would harm him and the children.

One night, one of the nine children disobeyed the temple rules and traveled beyond its borders. He is immediately accosted by a demon, and in order to save his own life, he tells the demon to eat the other children including Gyomei. After night fell, the child invited the demon into the temple after putting out the Wisteria Incense.

Four children were immediately killed by the demon and the remaining children insisted on escaping the temple, ignoring Gyomei's orders. His frail stature and blindness fueled the distrust the children had in Gyomei's ability to protect them, which led them to abandon him and ultimately getting killed by the demon.

In the end, only one child—the youngest, Sayo—had listened and stayed behind in fear of the demon.

Though it is unknown how the exchange went down, Gyomei had been cut across the forehead by the demon but soon reversed the situation and began attacking the demon mercilessly after realizing he had a "frightening amount of power." He went on and beat the demon's head until the sun arose.

Soon after, people came to the scene of the incident to help but due to a misunderstanding with Sayo saying "That man's a monster. He killed them. He killed everyone." the villagers thought she meant Gyomei and not the demon.

He was later imprisoned and charged with the murder of the seven children but Kagaya Ubuyashiki, a.k.a "Oyakata-sama", intervened and released him before he would've been executed for his false crimes.

Later on, Gyomei became a Demon Slayer and after only two months, the Stone Hashira.

Natagumo Mountain Arc
Gyomei was present with the other Hashiras when Tanjiro Kamado awoke.

Functional Recovery Training Arc
As the Hashiras discuss what to do with Tanjiro and Nezuko Kamado, Gyomei takes pity on Nezuko's form and calls her shoddy, saying it was a pity she was born into the world. Upon seeing Tanjiro's determination, he says that it is a pity how obsessed Tanjiro is and that they should kill him to release him from his suffering.

When Kagaya Ubuyashiki arrives, Gyomei kneels to greet him alongside the rest of the Hashiras and is the first to speak after the Oyakata announces that he has given his approval to Tanjiro and Nezuko. Gyomei says that despite the Oyakata's wishes he was reluctant to accept the pair.

Gyomei looked on as Sanemi attempted to test Nezuko with his blood and took pity on Tanjiro, who was being restrained by Obanai Iguro for being a weak child. After Nezuko refuses to drink Sanemi's blood and Kagaya declares that the pair may leave in peace, he commences the Hashira Meeting with Gyomei also in attendance.

Demon Train Arc
Gyomei appears at the very end of the arc, where it shows him receiving the news of the Flame Hashira, Kyojuro Rengoku's, death. Gyomei is visibly saddened, tearing up, and offers up a prayer.

Hashira Training Arc
Gyomei was present at the meeting held at the Ubuyashiki Estate in the aftermath of the attack on the Swordsmith Village. He stops Sanemi Shinazugawa from getting into a fight with Giyu Tomioka over the latter's refusal to participate in the grand training of all the Demon Slayer Corps in order to find as many people who were capable of manifesting the Demon Slayer Mark as possible.

When Tanjiro Kamado and Zenitsu Agatsuma arrive at Gyomei's training camp in the mountains, they are shocked to find all the trainees, including Inosuke Hashibira meditating under a waterfall. They turn around as Gyomei greets them and are shocked to see him standing tiptoe on burning ground while balancing logs and boulders on his shoulders.

Gyomei says that the most important thing is the body's core, the limbs and that his training was geared to strengthening the limbs. He explains that his training consists of only three things: training underneath the waterfall, carrying three logs on one's shoulders and pushing a large boulder through a nearby town. Inosuke, who had passed out underneath the waterfall and been resuscitated by Tanjiro, noted that he could sense that Gyomei was the strongest Demon Slayer and Tanjiro agreed with this, stating that the Rock Hashira was the only one who had a different scent from the others.

He is later seen pushing a giant boulder, bigger than the one the trainees were to use across the mountain. The training was quite difficult but also not compulsory, which meant quite a few of the trainees quit midway, left aghast at the effort required. Genya then reveals to Tanjiro that the chant Gyomei would repeat often was in fact a series of repetitive actions designed to maximise his focus and that allowed him to pull off feats such as pushing the boulders.

Meanwhile, Gyomei looked on at the two from a distance as Genya continued to explain that while chanting, both him and Gyomei would remember their anger and painful memories which made their heart rate and temperature rise. Later on, when Tanjiro successfully completes the Rock Hashira training but is left severely dehydrated, Gyomei pours water on him to save the boy.

He then continues on and says that due to completing his training and for his actions in the Swordsmith Village of prioritising the villagers over his sister he believes Tanjiro is a true swordsman. However Tanjiro states that the choice was made by Nezuko and not himself and thus he could not accept Gyomei's acceptance and upon seeing this the Rock Hashira is left further impressed at Tanjiro's honesty.

Saying that all his doubts are now gone, Gyomei states that he has fully accepted Tanjiro regardless of what the other Hashiras may say. He continues and reveals that when he was younger he raised nine orphan children in a temple and that one day one of the children broke their curfew and stayed out till after sunset, leading to the child being chased by a demon.

However, the child attempted to save himself and told the demon to eat all the children and Gyomei at the temple instead of himself, going as far as to put out the wisteria incense Gyomei would burn outside the temple at night to ward off demons. The demon snuck inside and immediately killed four of the eight children, Gyomei, who at the time was skinny and weak with a soft voice, attempted to protect the other four but three of them disobeyed his orders, believing their blind caretaker would be of no use. All three of them were also killed, leaving only the youngest, Sayo, alive as she hid behind Gyomei.

Determined to protect Sayo at all costs, Gyomei fought the demon and managed to pin it down and beat it's head in with his bare fists until dawn. He thoroughly disliked the experience of hitting another living being but persevered to protect Sayo.

As the sun finally rose the demon died and Gyomei who had lost many things that night had at least managed to protect her, however when people arrived and were shocked at the carnage, they asked Sayo what happened and said that the man was a monster who killed all the children and since the demon's corpse had disintegrated in the sun, everyone assumed she had been speaking of Gyomei and he was sentenced to death.

This was heartbreaking for Gyomei who had wanted for Sayo to be thankful to him for fighting for her, however he understood that the child would have been thoroughly confused due to all the chaos and would not have meant to unintentionally frame him. He was saved from execution due to an intervention from Kagaya Ubuyashiki and had been deeply paranoid of all people ever since, however he said that Tanjiro had now proven himself and that he would do everything he could to make Tanjiro would make no missteps in his life and pats the young Demon Slayer on the head, proud of him.