Seiichiro Ishima was born the first son to Norifumi Ishima (徳史 斐島 — "benevolent history," "beautifully patterned island") and Natsume Ishima (夏目 斐島 — "summer eyes," "beautifully patterned island"). They had two more boys after that, Shizuo (静夫 — "quiet man") and Sumio (澄雄 — "clear hero"). His father worked as a coal miner, and his mother works as a caretaker at the local inn.

If his parents taught him anything, it was to always shoulder the burdens of his two younger brothers, to take responsibility for them and protect them, and to work hard. If he had nothing else in life, he should just keep working. There is no easy way out of anything. There are no handouts. Life will be hard, but work and you will be rewarded. Natsume worked the hardest of any of the innkeeper's employees. When the host died, she named Natsume in her will as the benefactor and inheritor, and thanked Natsume for all her dedication and hard work. Norifumi worked the hardest of any of the miners. He would come in even with an injury, and so the shifty-eyed contractor who came to town looking to invest named Norifumi as the project manager. Such hard-working people always being compensated for the time they spent working.

Until they weren't.

Three days pass, and Norifumi doesn't come back. Neither do any of the other men. The contractor stops by every family's door to deliver the bad news with his head down and his voice quiet, as if he's afraid of being heard by anyone other than the wives and children of the men that were his responsibility, as if he's afraid of being heard by their ghosts. The contractor claims the mine collapsed, but Seiichiro has suspicions. There had been one collapse once before when he was much younger, and he distinctly remembers there so much rumbling that the town thought they'd been hit with an earthquake. Besides, he could recall his dad talking about how much sturdier the supports were this time around, now that they had the resources and the expertise. There's no way it was as simple as a mine collapse.

He gathers nine other children, including his closest childhood friend Yoshida Senshiro, all children whose fathers died in the alleged collapse. Being the oldest at fourteen years old, Seiichiro easily convinces the other children to come with him to investigate the mine at night when there was no one around. 

Nothing seems out of place. It's surprisingly empty for a mine that had supposedly collapsed in on itself a few days ago. Hadn't the contractor said something about it being too dangerous to go digging for the bodies so soon after? Yet no matter how deep into the mine they went, they couldn't find any sign of a collapse. With Seiichiro leading them, they get to a wide open area with a river running through, and are disturbed to find human remains scattered all over the place. Only then does it occur to them that they were being watched.

In one fell swoop, three heads are rolling on the floor, separated cleanly from their bodies. A horrific creature reveals itself. Seiichiro doesn't need to think twice, he yells at everyone to run. The other children are not blessed with Seiichiro's good sense of direction and general knowledge of the tunnels however, and he doesn't realize nobody has followed him until he's running out through the entrance.

He turns, ready to run back in, but is frozen in place by his fear. That thing had so many limbs, so many teeth. It can't have been human. What can he do against a monster like that? He waits for a long time for anyone to emerge, long enough that it hits dawn. Only then does a blade-wielding fiery-haired young man around his age show up to question him. Seiichiro explains what happened, and the young man, realizing with alarm that he was too late, hurries into the mine. Although he told Seiichiro to go home and be safe, Seiichiro stays to wait.

The young man emerges from the mine with minor injuries. None of the children are with him. As he explains that the demon escaped through the tunnels once it realized it was outmatched, Seiichiro only hears and understands one thing. His friends are all dead. He led these children to their deaths. It's his fault. All of it.

He cannot even bring himself to cry at that moment, even as the young man attempts to comfort him. He cannot feel anything but overwhelming guilt. Seiichiro begs the stranger to stay, he needs to do something, help someone. Hesitant at first, the young man wants to go after the demon, but at Seiichiro's insistence, he decides it'd be best to return to these mines with a team, and agrees to come back with Seiichiro to his home. There, the young man introduces himself as Kyōjurō Rengoku. He explains to Natsume that her husband was most likely killed by a demon in the mines, as were all of the men on the project, and that the contractor, who is suddenly nowhere to be found, most likely knew about it, or must've been a subordinate to the demon. He doesn't tell Natsume that Seiichiro went to the mines or took anyone else there. He doesn't tell the townspeople either. Seiichiro is thankful for that.

As Kyōjurō recovers, he tells Seiichiro about demon slayers, and he remarks that Seiichiro has such a fire burning in his heart, that if he truly wanted to make up for what has happened, that he should grow strong enough to take the Final Selection and then make sure it never happens to anyone else. He warns, however, that the life of a demon slayer is a hard one, and it is perilous. He might never come home again. Seiichiro goes to bed with that in mind.

After the funeral, as Kyōjurō prepares to meet up with a team of slayers to chase after the demon, Seiichiro begs him to take him along and teach him how to fight, which is met with a forceful, "No way!" Seiichiro insists, promises to prove he won't be a burden, cracks his knuckles, and... massages Kyōjurō's shoulders? Eh? How is that supposed to prove anything? He's not looking for a massage therapist! Get lost!

Except Seiichiro doesn't give up. In fact, without telling anyone, he follows after Kyōjurō and tries again, randomly popping out from God knows where and demanding Kyōjurō make him his student. When this inevitably fails, Seiichiro takes a different approach, and learns from Kyōjurō from afar, imitating his movements and breathing style. He doesn't learn the Breath of Flames. No, instead, he creates his own style, the Breath of the Tiger, as he supposedly spent the later half of his training in isolation with a tiger as his mentor. In three years, Seiichiro is ready to take the Final Selection.

During the trial, he meets a girl a few years younger than him named Yukiko Haruta, who he makes friends with. He promises her that he will try to protect everyone there. This proves an impossible feat. Though casualties are limited to only a couple competitors, Seiichiro cannot help but feel that it is yet again a failure on his part to protect anyone. He and Yukiko stand side by side to receive their ranks and Kasugai crows, and when he realizes he and Yukiko's mission are taking them in the same direction, he follows after her and offers his assistance. It doesn't take much longer for them to grow closer, and they run into each other many times after that.

Notes:

Notes:

STATS

Full focus breathing: This skill is used to generate an oxygen supply in the user's blood and strengthen their muscles, and gives them the power and stamina to combat demons.

Lungs of steel: Having lived in a mountainous region for his whole life, Seiichiro is able to handle extended periods of time in low oxygen environments with little effect on his capabilities. This also allows him to hold his breath for extended periods of time. A portion of his regimen always takes place in a low oxygen environment.

Immense strength & durability: Seiichiro claims he sparred with a tiger that kept following him through his training years, and that gave him the strength, speed, and durability necessary to finally beat it. Oddly enough, no one can prove whether this tiger actually existed, but it was through this tiger he trained his muscles to withstand superhuman levels of pressure and stress.

Seiichiro's blade is designed by the swordsmith Gakuto Hozaki (岳登 保崎 — "mountain," "ascend," "protect," "cape"). When Seiichiro first held his blade up to the sunlight, it turned a brilliant crimson. The blue handle is decorated by gold accents, and a red flower and tassel. The sheath is dark blue.

BREATH OF THE TIGER:

This particular breath style was developed after Seiichiro failed to learn the Breath of the Flames style and branched off. It is his own personal style, and focuses on striking with overwhelming power and pinpoint precision.

First Claw: Flashy Stripes (壱の爪派手な縞, Ichi no tsume: Hade na shima) — The user slices the target in a ribbon-like fashion, twirling around them and leaving them with wounds like stripes. This technique can also be used in a defensive stance to deflect attacks.

Second Claw: Thunderous Roar (弐の爪雷鳴, Ni no tsume: Raimei) — The user kicks off a surface with such force it produces a thunderclap, and sends the user spinning in a vortex to shoot right through the target.

Third Claw: Sharp Sense (参の爪鋭い感覚, San no tsume: Surudoi kankaku) — The user uses the blood of a target to track them.

Fourth Claw: Bloodlust (肆の爪殺気, Shi no tsume: Sakki) — The user uses their own blood to extend the blade's reach, making for a powerful but risky long-ranged attack.

Fifth Claw: Shred to Nothing (伍の爪無に寸断する, Go no tsume: Mu ni sundansuru) — The user unleashes a series of slashes that are so quick, they seem to be dealt simultaneously, and shreds a target to ribbons. This technique can also be used in a defensive stance to cut apart oncoming attacks.

Sixth Claw: Mountain King (陸の爪山王, Roku no tsume: San'nō) — Starting from a high stance, the user slashes down, striking the target with such force that it splits the ground upon impact. This technique cannot be blocked. It is incredibly taxing on the user, and immediately renders them immobile for up to an hour.