Obanai Iguro/Synopsis

History
Obanai was born into a clan of thieves who lived off an excessive amount of riches that had been stolen from those who had been killed by the Demon that they worshipped. This Demon took the form of a woman with a snake-like lower body and face, and had a great taste for newborn babies, which is what the family sacrificed to her in exchange for their riches.

As the first boy born into the family for 370 years, Obanai lived out all of his days locked in a wooden cage being spoiled by his family, who insisted on bringing him so many plates of rich food that he became sick of it. The Serpent Demon also took a liking to Obanai, since he was a rare male child with multicolored eyes, and would spend the nights slithering around his cage in the dark and staring at him, which frighted Obanai so much that he couldn't sleep.

When he turned twelve, Obanai was finally allowed to leave his cell. He was taken into a room that appeared to be an expensive shrine, and put face to face with the Serpent Demon for the first time. The Demon remarked that Obanai was very small, and wondered to herself if she should wait to eat him until he was a little bigger. Deciding that she would, she organized his mouth to be cut wider, all the way to his ears, to match hers. The blood that he shed from this procedure was collected into a chalice that she then drank from.

Back in his cell with his wound now covered by a bandage, Obanai began only to think of escape and survival. He began to scratch away at the lattice of his cell with a stolen hairpin during the night, and by day he would be in a constant state of worry for fear that they could discover it. The only being he could trust was the snake that had strayed into his cage, Kaburamaru.

After a while of doing this, Obanai finally made his escape. As he ran, the Serpent Demon began to pursue him, almost catching up to him and killing him before the Flame Pillar by chance came to his rescue. Because of Obanai's escape, the Serpent Demon had killed all fifty of his relatives and the only surviving member of his bloodline was his older cousin, who had also been saved by the Pillar. When they were reunited, however, Obanai's cousin rejected him, blaming him for their family's death.

Due to the effect his cousin's words had on him, Obanai joined the Demon Slayer Corps and decided to turn all his rage onto Demons, figuring that he'd feel like a better person if he risked his life for others.

Functional Recovery Training Arc
As the other Pillars discuss what to do with Nezuko Kamado, Obanai, lounging in a nearby tree, inquires how they would deal with Giyu Tomioka's breach of the Demon Slayer conduct. When Tanjiro insists Nezuko would never harm humans despite turning into a demon, the Snake Pillar tells him to not make such reckless remarks, as the girl would obviously protect her family, and as such he cannot trust anything the young Demon Slayer says. Soon afterwards, he notes with interest that even though Giyu had interjected to stop Tanjiro and Sanemi Shinazugawa's brief scuffle, the former had still managed to land a blow on the latter.

Upon Kagaya's arrival, the Snake Pillar descended the tree and bowed in respect to his superior. He then reiterated his distrust for Tanjiro and Nezuko when his leader asked for all of them to accept the two, stating that they all hate Demons. He subsequently listened as Kagaya's attendant read Sakonji Urokodaki's letter concerning Tanjiro and Nezuko, and how Sakonji, Tanjiro, and Giyu would all slit their stomachs should Nezuko ever attack a human. When Kagaya revealed that Tanjiro had come into contact with Muzan Kibutsuji, a mess of questions and arguments broke out, but Obanai and the other Pillars are silenced by a simple gesture from their leader.

After Sanemi purposely injured himself to bait out Nezuko, Obanai reminded his fellow Pillar that he would have to get out of the sunlight in order for her to appear. He then swiftly pinned Tanjiro to the ground to prevent him from interfering, maintaining his hold despite Shinobu's protests.

Tanjiro continued to struggle heavily, but before Obanai could do anything further to him, Giyu grabbed his hand, allowing Tanjiro to escape from his grasp. Obanai angrily removed his hand from Giyu's grip, demanding to know what he is planning. The Snake Pillar was then chastised by Kagaya for his treatment of Tanjiro, causing him to reluctantly agree to refrain from such methods in the future.

Demon Train Arc
Obanai stands on a rooftop alone after learning of Kyojuro Rengoku's death, staring into the distance as he expresses his disbelief.

Entertainment District Arc
Obanai appears at the very end of the mission, after Daki and Gyutaro were already defeated. He partially congratulates his fellow Pillar for the victory against the Upper Moon, but greatly emphasizes the grievous injuries sustained against the "weakest" Upper Moon. He pressures the Pillar into remaining with the Demon Slayers, pointing to the vacancy left by Kyojuro's death and the increase in losses of lower-ranking Demon Slayers. Much to his surprise, Tengen maintains that the younger Demon Slayers—especially Tanjiro—were "definitely growing."

Pillar Training Arc
Obanai was present at the emergency Pillar meeting at the Ubuyashiki Estate. After Sanemi complains about not encountering any Upper Moons himself, Obanai comments that bumping into one is rare and then asks Mitsuri and Muichiro how they're doing. Amane Ubuyashiki arrives and apologizes for the absence of her husband. Obanai and the other Pillars bow to pay their respects. Amane informs the Pillars about the Demon Slayer Marks and asks Mitsuri and Muichiro to tell about how and what are the conditions to acquire one. Obanai is visibly suprised when Amane mentions that these marked Demon Slayers were close to defeating Muzan. When Mitsuri tries to explain how she got her mark, Obanai expresses embarrasment. After that Muichiro tells about his theory on the conditions to acquire a Demon Slayer Mark to Obanai and the others. When Giyu comments that he envies those simpletons who think gaining a mark is simple, Obanai glares at him, but says nothing. When the meeting ends, Giyu prepares to leave and tells the remaining Pillars to make future plans to acquire marks by themselves. Obanai angrily retorts that Giyu does not understand his position as the Water Pillar and asks if he plans to get a head start on them. Giyu simply states he is not like them. The other Pillars protest Giyu's decision as well, but before the situation escalates into fighting, Gyomei slams his hands together making Obanai and all of the others freeze in their tracks. Gyomei states he has a proposition.

This proposition turns out to be special training session called "Pillar Training". Obanai is put in charge of teaching proper sword techniques. When Tanjiro arrives to his training, Obanai states he has been waiting for him. Tanjiro tries to introduce himself, but Obanai tells him to shut up. He tells Tanjiro that he has heard about him from Mitsuri, but warns him that unlike her he is not a nice person. Obanai reveals that his training consists of swinging your sword while avoiding obstacles. He leads Tanjiro to a room where he has tied up all the students, making them into a sort of obstacle course. When Tanjiro questions Obanai about the students and what crimes have they committed to be tied up, he menacingly answers that they committed crimes of weaknesss, forgetting, taking their time and pissing him off. For the next four days Obanai trains Tanjiro to hit him while not hitting any of the tied up students. During the fourth day Tanjiro finally manages to slice off the haori Obanai was wearing, thus completing the training. Immediately afterwards, Obanai tells Tanjiro to "get the hell out of there" and to never talk to Mitsuri again.

Dimensional Infinity Fortress Arc
Obanai rushes to the Ubuyashiki estate after hearing the call that Muzan was there. However, Kagaya blows up the estate before he or the other Demon Slayers can arrive, killing himself, Amane Ubuyashiki, Nichika Ubuyashiki, and Hinaki Ubuyashiki while heavily wounding Muzan. Gyomei Himejima is the only Demon Slayer to know of this plan beforehand, as Kagaya knew the other Pillars would oppose his decision. After seeing Muzan, Obanai readies himself to unleash his first form on him, but Muzan transports him and the rest of the Pillars into his Dimensional Infinity Fortress. Obanai runs into Mitsuri and kills a group of Demons that she was fighting.

Later, he and Mitsuri encounter Nakime. Mitsuri jumps off a wall and lunges at the Demon, but Nakime uses her Blood Demon Art to erect a wall and send Mitsuri falling. Obanai jumps in and grabs her, getting her safely to another of the Fortress' pillars. As Nakime starts manipulating the Fortress in the hope of crushing the pair, Obanai is easily able to dodge. After Mitsuri tries to attack and was sent tumbling out of a window, Obanai tries to use his second form to decapitate the Demon. However, he too is sent out a window, expressing in a frustrated fashion how much of a pain Nakime's Blood Demon Art is to deal with, despite it not being very deadly.

After fighting Nakime for a while, Obanai and Mitsuri are informed of Muzan's revival and receive orders to group up with the other Pillars. After hearing how Muichiro and Genya sacrificed their own lives in order to defeat Upper Moon One, Obanai berates himself for not contributing as much to the battle as others have. He remarks how Nakime's Blood Demon Art makes it impossible for him to settle the fight. He thinks he must somehow find a way to get out of this situation.

With the help of Yushiro and his Blood Demon Art, Obanai and Mitsuri are able to get away from Nakime and they arrive to assist Giyu and Tanjiro who were fighting against Muzan. Obanai appears by Tanjiro's side, who was almost killed by Muzan, and tells him that he is a burden that needs to fallback. Tanjiro ignores Obanai's advice and instead says he is glad that Obanai is still alive. Obanai tells him to worry about himself first instead of others.

As Muzan grows furious over Nakime's failure, the battle between the Demon Slayers and Muzan resumes. Obanai uses his first form to attack Muzan, who blocks the attack. As the Dimensional Infinity Fortress starts to rapidly change due to Yushiro manipulating Nakime, the floor below the Demon Slayers and Muzan disappears. Obanai falls and lands on his feet on the platform below. Muzan continues his attacks which Obanai avoids. Obanai worryingly thinks that they need to get out of the Fortress before it crumbles, or they will all die. As Mitsuri is caught off guard by the platform rising, Muzan uses this chance to attack her, but Obanai jumps in to get her out of the way of Muzan's attack thus saving her life. The Fortress crumbles, but due to Yushiro, Obanai and the others are safely transported to the surface.

As the battle moves on to the surface, Obanai emerges from the wreckage of the Fortress. A crow informs the combatants that an hour and a half remain until morning. Muzan blasts the wreckage aside with his newly manifested Spine Whips and taunts the Pillars to try keep him there for that long. Muzan starts attacking once again. Obanai uses his third form and manages to get close to Muzan. He starts cutting of his head and remarks that even if Muzan does not die from getting his head cut off at least they could weaken him by slicing him to pieces. This does not prove to be the correct move as Obanai is shocked when his attack does nothing to Muzan. Obanai and the others realize Muzan is able instantly regenerate to the point that it is useless to try to weaken him using this tactic. As they figure out their mistake, they also realize they got too close to Muzan and are about to be killed by him. However, before that can happen Obanai and the others are shielded by lower ranking Demon Slayers who use their own bodies as shields. Obanai is grabbed by one whose entire lower half of body is sliced off. He is visibly shocked by this sacrifice but says nothing. Muzan reveals that just getting scratched by him once is a death sentence as he points to Tanjiro who is laying down on the ground lifeless.

Obanai and the two other Pillars continue their fight against Muzan. Obanai remarks on how he can't continuously breathe even with the three Pillars there due to Muzan's speed. As the battle rages on, Obanai is slashed once in the head and once in the shoulder by Muzan, but he continues to fight. Mitsuri, however, is immediately affected by Muzan's attack and falls over. Obanai yells her name, but she tells Obanai and Giyu to protect themselves instead. Before she is killed by Muzan, Gyomei appears and deflects the attack with his flail. At the same time Sanemi appears and uses the opportunity to slice Muzan in half and sets him on fire. The battle continues with Obanai and the four other Pillars joining forces.

As the battle continues and the Pillars keep getting pressed on, Obanai thinks about how he can't even shield Gyomei from attacks anymore and soon he will be unable to move himself. When Mitsuri is hit by another attack which forces her to the ground, Obanai is worried and hurries to her side. As Muzan prepares to slice both of them, the three other Pillars shield them while Obanai carries Mitsuri to safety. He entrusts her to a Kakushi and tells them to find Yushiro. Mitsuri tries to argue that she can still fight, but Obanai tells her she has done enough. He tells the Kakushi to take care of her and immediately rushes back towards the battle, ignoring Mitsuri's pleas. As he runs, he starts to ponder about how many people would still be alive if Demons didn't exist. He wonders how things would have been if he and Mitsuri had met under more peaceful circumstances. However, he dismisses this thought and considers it impossible. He thinks back on his childhood and how he wants to die defeating Muzan in order get rid of his family's impure blood. Obanai hopes that when he dies and if he and Mitsuri both reincarnate into a world without Demons, he can tell Mitsuri that he loves her.

After joining the battle once again, Obanai manages to cut off Muzan's arm by kicking and slashing the arm at the same time. Muzan notices that it is hindering his instant regeneration but remains unworried as it only buys them a miniscule amount of time. Obanai thinks to himself that Muzan's arms have mouths that have a dangerous inhalation ability. In order to avoid it he has to make huge movements that take up a lot of stamina. He proclaims to himself that they cannot afford to lose as he does not want all of the sacrifices thus far to be in vain. As the battle progresses Giyu loses his grip on his sword, but Obanai shields him from Muzan's attack and at the same time Sanemi tosses a new sword to Giyu. After 16 minutes of fighting since coming to the surface, Muzan is surprised by how long the Pillars have managed to last. As Muzan's poison is starting to affect the Pillars more and more, Obanai realizes that the poison is affecting him more than others due to his small body. In that moment, Chachamaru appears carrying antidotes to Muzan's poison. Obanai and the Pillars receive the antidotes, but Chachamaru is killed in the process. As Muzan grows furious, Obanai considers that he needs to find a more effective way to attack. He believes that he has contributed the least out of everyone. Thinking back on the report that Muichiro had turned his blade red during his final moments, he speculates that by gripping the sword as hard as you can, you can increse the temperature of the blade and thus turn it crimson red. In that moment his Demon Slayer Mark appears, and he grips his sword turning it crimson red.

After turning his blade red, Obanai's first thought was that he had made a mistake. He believes he was too eager to turn his blade red and used too much strength to do it which almost made him faint due to lack of oxygen. Sanemi yells Obanai's name as Muzan sends an attack towards Obanai. In that moment Obanai is unable to do anything, but to think of the fact that if even one of them falls they will no longer be able to break Muzan's attacks. In the last moment before the attack hits Obanai is thrown into the air by invisible Inosuke. Muzan realizes three new opponents have joined the fight, Inosuke, Zenitsu and Kanao, but they are using Yushiro's Blood Demon Art to render themselves invisible. As Muzan was figuring out what was happening, Obanai landed back on the ground. Using the trio's distraction to his advantage he slices Muzan's hand off with his red blade. The sliced body part regenerates much slower which inspires the combatants to continue fighting even harder. Obanai once again attacks and use his third form against Muzan. This allows the other Pillars to get a chance to turn their weapons red in their own respective ways. A crow informs that one hour and three minutes remain until dawn, meaning the battle has raged on between the Pillars and Muzan for 27 minutes since they came to the surface.

Obanai, the other Pillars and the trio continue pressing on Muzan scoring several hits. Gyomei accesses the See-Through World and finds out Muzan has multiple hearts and brains. Gyomei yells to Obanai to focus on Muzan's body and tell him if he can see through it. Obanai tries it and sees through it for an instant which confuses him. In that moment Muzan unleashes his Leg Whips, which catch Obanai and the others off guard. Obanai is hit and sent flying. He hits a wall and is rendered unconscious and bleeding.

Obanai quickly gets up and saves Tanjiro, who was fighting Muzan by himself, from death. Obanai, having grabbed Tanjiro, swiftly dodges Muzan's attacks. Tanjiro is glad to see him, but is quickly horrified when he realises Obanai's face is mutilated. Obanai explains that the earlier attack by Muzan's Leg Whips was the one that injured him. He tells Tanjiro to stop worrying about him, because his right eye was always bad and he could barely see out of it and because of that he instead relies on his snake Kaburamaru to see. After confirming Tanjiro had recovered, he lets go of him. Muzan, who had given chase to them, attacks Obanai, but he is able dodge and block all of the attacks thanks to Kaburamaru. Tanjiro and Obanai combine forces to attack Muzan both using their respective techniques. While Obanai and Tanjiro continue to dodge and attack, Muzan's body suddenly changes revealing old scars. Tanjiro realises these must be from Muzan's battle against Yoriichi and they might be his weakpoints.