Izō Aiba

is a male delinquent from South Chinpira High School with a corresponding reputation to Tatsumi Oga as a "child-carrying gang leader".

Appearance
Izō is a tall, lean-built young man. He has black eyes and blonde hair that is slicked back, save for a tuft of hair that falls over his forehead.

He wears a light tank top under his school’s uniform and carries his younger sister in a small sleeping bag attached to a strap on his back. When not in uniform, Izō is seen wearing very casual clothing.

Personality
Despite being the leader of a bunch of vicious thugs, his personality is strikingly kind and calm. He is very easy-going, often joking around in a friendly manner, and even when shown with hostility. Izō does not react in the same way, easily shown by the numerous occasions where Nene Ōmori hit or insulted him yet be responded in kindness. He is also very honorable; when he loses to Oga in their battle, while frustrated that he lost, Izō was able to accept what had happened and showed no qualms towards Ishiyama thereafter, showing his end of the promise between their schools.

Still, his easy-going and relaxed ways are backed up by immense physical power which makes him rather tough and intimidating at times. He commonly attacks his underlings, usually to demonstrate his actual control of power on his school, but still holds care and loyalty for them.

Field Trip Arc
Izō enters the hotel that South Chinpira High School is residing at during the school trip.

He groans that riding on the bus makes him sick, before noticing that his group of underlings are having a feud with Ishiyama. When they argue, Izō takes action and punishes one of his subordinates, going on to say that "Chi" is going to wake up.

He introduces himself to the Ishiyama students and apologizes for his school's behavior. Izō then notices Oga and asks for the latter's name, but instead is told that he is "Madason". He is then approached by Aoi Kunieda and immediately becomes smitten. Izō advances on her but is unsuccessful thanks to Nene's interference; still, he is able to hear Aoi's name, but only her first.

Izō later walks around the hotel in search of Aoi's room but meets Oga instead. Both converse and eventually begin arguing, though when Izō notices how flustered his younger sister is upon seeing Baby Beel, he realizes that she's fallen for Oga's own "son".

Izō and his younger sister, whose name is soon revealed to be Chiyo, later accompany Oga and Baby Beel to the Red Tail's hotel room. He claims that he's only there as a part of the supposed agreement to visit Aoi later that day, and in regards to the situation with Chiyo and Baby Beel, says they're on a playdate. As Nene starts beating him up for his carefree nature, Izō is told that his and Aoi's astrological signs are suitable with one another, pleasing him. Afterwards, he is scolded by Nene for being so lenient with his underlings, but as usual, Izō brushes it off. However, he adds that he doesn't want South Chinpira or Ishiyama to fight with each other. He suggests that he and Aoi go out, which she agrees to do, under the condition that it's to strengthen school ties and that Oga accompanies them. He reluctantly accepts the offer.

Aiba brushes off the matter, sure that Ishiyama wasn't directly responsible for everything. He then sees Koma face-down in the ocean (due to being flushed down the toilet by Aoi earlier). Izō rescues Koma who then introduces himself to the two students of South Chinpira. He recognizes Izō as the person interested in Aoi and uses the situation to his advantage. Koma says how he's a god and that, because Izō saved him, he'll grant the teenager a wish just as long as it's related to love. Izō ponders the wish for a brief moment.

Izō eventually arrives, late, and sees Hilda. He takes Oga aside and punches him for bringing along two girls. He angrily asks who he actually likes and when Oga hesitates, Izō and Oga talk privately. Izō reveals that he knows about the supposed relationship between Oga and Hilda. He wants Oga to be fair to Aoi with her feelings, but due to his density.

Izō is told that Kanzaki beat up some of the South Chinpira students, then berated by Kanzaki himself for not being as authoritative over his underlings. Instead of turning to Kanzaki, Izō attacks Shigeharu and sends him flying into the ocean. Tōjō arrives and challenge Aiba to a fight, claiming to be bored. Izō recognizes him but decides not to fight, as he believes it's pointless and because he's on a "date" with Aoi. Tōjō tries to think of a reason to fight but fails, so he attacks Izō anyway. Izō not only dodges but manages to counterattack. Moreover, this action causes him to become angered. Aoi then walks up to him and reminds him of the deal he'd made. Izō tries to explain things but Aoi walks away. Izō and Aoi visit Okinawan shops and Oga considers buying ryukyu glass for his sister. Izō becomes jealous that Oga is spending time with Aoi and says that they'll have to fight each other to settle their score.

Much later, he finds his fellow schoolmates and learns from one of them about what had happened, angering him.

Aiba suddenly enters the room and yells out that he needs to talk with the Tōhōshinki. He goes to the beach with Oga and Baby Beel, as Hilda and Furuichi wait nearby. The Ishiyama Tōhōshinki soon arrive, annoyed at having been called so early in the morning. Izō is surprised to learn that Aoi is actually Aoi Kunieda, former leader of the Red Tail. Happy, he asks if she can be his woman causing Aoi to fluster in embarrassment. He challenged Hidetora Tōjō to a fair battle, as he wants to end their schools' feud once and for all. However, he is told that the actual leader of Ishiyama is Oga, who is the same person that Izō's known all along. Izō walks up to Oga, now realizing that he's the child-rearing gang leader infamous in Ishiyama. Seeing Izō's expression, Oga decides to fight him. However, Izō notes that Oga isn't really serious about their fight and wonders if he actually is stronger than Tōjō. But then, Oga attacks him with a powerful punch. Izō manages to dodge it, causing Oga to hit the ground instead and create a large crater in the sand. Izō notes on how interesting the fight's becoming.

As seawater pours into the crater, Izō says how the rumors he heard about Oga appear true. Izō turns to Aoi and bets that if he wins, she should go on a date with him, surprising her. Hearing this, Izō goes to Oga and says how excited he is about the fight. Izō holds his hand up in position to flick Oga's forehead. Remember what happened earlier, Oga does the same thing to Izō. As the others remark on the situation, the two delinquents agree to attack at the same time once they receive a signal. Izō and Oga flick each other simultaneously and begin fighting with punches and kicks, becoming evenly-matched. Izō shows that he's impressed with Oga's strength and decides to use his full power, drawing out slight Demonic power. A beastly Koma suddenly appears behind Izō, surprising both Oga and Aoi. Koma states that he plans to get revenge on Aoi for flushing him down a toilet earlier, doing so by defeating Oga and making her go on a date with Izō, who is unaware of Koma's presence. Izō and Koma end up falling down on the sand, both utterly defeated.

As he lies on the sand of the beach, quite exhausted, he compliments and praises Oga for his brute power, even adding how he took down a "God". A smiling Izō casually says how frustrating things are since he lost the fight, thus, losing his final chance with Aoi.

Much later, at the airport, Aoi is approached by Izō and they talk for a while. He says that he's given up on her, but only until she comes back. Aoi laughs in amusement and admits that she likes a certain someone, intriguing Izō. Afterwards, he joins the students from Ishiyama in a group photo to take as a memento of the trip. With the photo taken and their planes set to leave, Izō returns home with his classmates.

Powers & Abilities
Izō is shown to be incredibly strong for a Human teenager, even being compared to the same strength as Oga. A signature trait about him is his raw power. Izō is able to flick one of his fellow students with just his finger, yet send him crashing head-first into a wall pillar. On a similar occasion, Izō has sent the aforementioned student several meters across the ocean with relative ease.

It is also observed by both Hilda and Oga that he shows a slight presence of Demonic aura.

Chiyo Aiba
Izō cares a lot about his younger sister and often carries her on his back, which unintentionally led to his reputation as a "child-rearing gang leader". He describes her as being precocious and, possible because of it, often lets her do things independently such as take Baby Beel closer to the dolphin attraction at the aquarium in Okinawa.

Tatsumi Oga
Izō is quite friendly towards Oga. He became interested in him upon learning that Oga was also "child-rearing" and commonly refers to him as "Madason", for that was the name Oga initially gave him. Izō became more interested and seemingly angered after learning Oga's true identity. After Oga defeated him, Izō returned to being his usual amicable self to Oga, who never seemed interested in Izō in the slightest.

Aoi Kunieda
Since meeting her, Izō has become infatuated with Aoi and has repeatedly flirted her. He is shown to care about her to a degree, becoming angered with Oga when he sees that Oga is (unintentionally) toying with her feelings for him. Upon realizing that she is the same Aoi Kunieda of the Tōhōshinki, he was neither angered nor upset; instead, he was happy and found himself more in love with her than ever. Before she left Okinawa, he told her that he's given up on her for the time being but will return to win her love if they ever meet again.

Hidetora Tōjō
Izō knows who Tōjō is and seems well-acquainted with him, being very casual during conversations with him. However, this only appears to be one-sided. Tōjō finds Izō to be strong and has challenged him to a fight before, even going as far as to make a pathetic excuse to fight when Izō refuses to.

Quotes

 * (To Aoi Kunieda): "My name is Izō Aiba. Is it okay if I come by your room later to play?"