This was because more limbs were required to accurately depict the movements of the legendary swordsman. No one knows the strength of Yoriichi Tsukiguni like his brother does. It is highly possible that Kokushibo manipulated his flesh and decided to have 6 eyes to have better vision.
Another theory suggests that he obtained the eyes simply through his desire to perceive the Transparent World in order to defeat his Demon Slayer brother, Yoriichi. Regardless of how he got his six eyes, they simply add to the terrifying arsenal that Kokushibo has under his belt.
His middle set of eyes has the daiji of "Upper Rank ( 上 じょう 弦 げん , Jōgen?)" and "One ( 壱 いち , Ichi?)" engraved in them while his other sets of eyes feature black pupils and a cracked pattern in the irises.
Sadly though even with six eyes, Kokushibo couldn't see the love that Yoriichi had for him until the end. Kokushibo's character was quite contradictory. Even with the amount of times he felt jealous of his brother, or how he stated he hated his brother.
The number one ranked Kizuki is the terrifying Kokushibo, whose six orange eyes mark him out as one of Demon Slayer's most striking character designs. He also bears a major resemblance to Tanjiro thanks to his red-tipped hair and flame-marked face, although that appears to be a coincidence.
The divine nature of sun breathing: Tanjiro's link to Yoriichi. The flashback also reveals that Yoriichi had a twin brother named Michikatsu Tsugikuni, who became a Demon Slayer but later turned into a demon and became Upper Moon One, Kokushibo.
Some of the characters who have faced Yoriichi and survived
This happened due to a treatment that had gone wrong. While he was being treated, Muzan killed the doctor because he was not able to observe any results.
As the battle reached its climax, Tanjiro was able to knock the doll's head off and discovered that the mechanical body contained a mysterious sword. Fans of the manga will recognize this sword as the one carried by Yoriichi Tsugikuni himself.
Demon Slayer season three spoilers follow
We watched Tanjiro spend most of the episode battling against an extremely cool mechanical doll, the Yoriichi Type Zero, under the tutelage of young swordswmith Kotetsu until the pair were surprised by a treasure hidden inside.
Many Kimetsu no Yaiba fans deduce that Yoriichi is Tanjiro's father, but that isn't the case. However, Yoriichi is responsible for teaching Tanjiro's ancestors the Hinokami Kagura, which has been passed down for generations.
The only reason why Muzan hates Doma is because of his insincerity and lack of emotions. This might sound odd, expecting emotions from a demon, but even a demon has to retain such tendencies to keep things in order.
Appearance. Yoriichi has slightly more slimmer bangs and wavier hair than his brother. His hair is colored in black to red and burgundy in the anime, like Tanjiro Kamado.
Yoriichi was the very first user of the Breathing Styles and he came up with Sun Breathing, a powerful breathing form that is known to be the most effective against demons.
He was so afraid of death that he allowed himself to become a Demon and therefore immortal. This makes him one of the few known Demon Slayers to defect and turn to become a Demon. In doing so he abandoned a wife and two children, though unlike many new demons he did not slaughter the vestiges of his human family.
Seeing that he still thought highly of him even after his transformation, Kokushibo couldn't help but shed a tear. Unable to truly sever his bond with his brother, he decided to keep the flute.
The divine nature of Sun-Breathing is the reason Tanjiro Kamado bears such a striking resemblance to Yoriichi, even though the two are not related.
After Yoriichi saved the Kamados from demons back in the Sengoku Era, he taught the Hinokami Kagura to Tanjiro's ancestor. Almost every generation of the family attempted to perfect this Breathing Style, but Tanjiro is the only one truly "chosen" to wield the technique.
Since the mechanical doll was based off Yoriichi Tsugikuni, it had to be given six arms to recreate his swordsmanship style, the doll was also able to execute 108 various moves and is said to be stronger than an average human.
Yoriichi was powerful enough to defeat Muzan; however, he couldn't do so. That's because he couldn't come down to kill his twin brother, leading to his own demise. Even to this day, Muzan is scared of Yoriichi.
So now, Tanjiro, the protagonist of the Demon Slayer series, has access to a sword that belonged to the best swordsman to have ever lived. This sword was in bad condition, but Hotaru Haganezuka took it upon himself to restore the blade to its former glory.
This is the sword that appeared out of the neck of the doll at the end of Demon Slayer season three's second episode. In the manga Tanjiro has the sword repaired by Haganezuka, who he initially traveled to the Swordsmith Village to find.
When Yoriichi moved forward to kill him, Muzan used the remains of whatever power he had left and divided his body in a powerful burst. Though Yoriichi was able to destroy nearly all the fragments, the pieces he missed escaped and allowed Muzan to regenerate again.
Yoriichi would be able to defeat every single Hashira on this list without breaking a sweat. Such was his reputation and skill in swordsmanship.
Yoriichi lived till old age, likely due to him having been born with the mark and thereby avoiding the curse altogether. Gyomei on the other hand did not activate his Mark until he was already older than 25. It was speculated that when he awoke his Mark during battle that he would likely die within hours.