Banba is described as a tall, lean man usually dawning a white sweater with old jeans. The novels describe him as having messy, unkempt, black hair and brown eyes however, illustrations show him with dark brown hair. Despite his noted height, he is usually hunched over.
Banba has a gentle and laid-back personality, usually not caring for his appearance or the cleanliness of his residence, much to Lin's annoyance. He is usually patient and understanding in most scenarios, but when it comes to baseball he can be overbearing, especially to the novice of their team, Xianming Lin.
With his mother shortly dying after giving birth to him, Zenji Banba grew up single-handedly by his adoptive father, Kazuyoshi Banba. Banba had a normal and peaceful childhood until one day during high school he arrived home to find an unknown man stab his father. Having just came back from his baseball club activities, Banba attempts to attack the intruder with a baseball bat on hand only for it to be taken from him and used on him instead. He loses consciousness, and by the time he regains consciousness another man wearing a Niwaka mask knocks out his attacker before calling the ambulance to get Banba medical treatment.
Banba wakes up later and is informed that while he managed to survive almost certain death his father did not pull through. He asks one of the police detectives that frequently visited him for questioning, Shigematsu, about the Niwaka mask wearing man he saw. Determined to find the man, Banba drops out of school and starts working at a host club in the Nakasu area, where the man in question is noted to shown up on occasion.
Mayor Harada Scandal Arc Edit
Banba meets with Shigematsu in Kego Park, who informs him of the recent death of his colleague, Takeda. While it was officially ruled a suicide, the two conclude he was most likely killed for investigating Mayor Harada and his connections. After Shigematsu gives Banba a photo left behind by Takeda, he asks him to figure out the circumstances behind the detective's death. The photo shows the Mayor in the company of two unknown men and a woman. Suspecting the woman in the photo to be a killer, Banba decides to consult with the informant, Enokida.
He learns that the woman with the mayor is Reiko Asakura, an assassin and former Murder, Inc. employee, who specialises in killing people with poison. However, Banba points out that it is unlikely she was responsible for Takeda's death, and asks Enokida if he can dig up any information on anyone who specializes in strangulation. With this, Enokida swiftly pulls information on another former Murder, Inc. employee, Hisashi Ivanov. Before Banba leaves, Enokida gives him a GPS-equipped redback spider model listening device.
On his way home, Banba runs into Yamato and ends up questioning him about the people pictured with the mayor. Yamato recognizes one of the men in the photo as Takashi, an employee of a club called Miroir, so Banba heads there in an attempt to find out more. However, when questioned, the manager claims to not even remember if the mayor had ever been there. Banba is confident he is lying and leaves knowing he succeeded in confirming their relations and catching their attention.
After updating Shigematsu on the situation, Banba returns home to find himself being greeted by an unexpected visitor - a hitman named Xianming Lin, who came in order to protect him from other hitmen the Kakyuu Group will send. He explains that while he had originally been sent to kill Banba, protecting him was his way of getting back at his employer over a disagreement. Since Banba decides to humor Lin by staying inside and out of harm's way, he gives Lin the task of going out to buy some mentaiko. After returning to the office, the two of them eat some tonkotsu ramen and watch a baseball match.
The following morning, Banba leaves the office while Lin is sleeping and meets up with Enokida in order to find out more about Lin. After retrieving this information, he returns home to find Lin, who has tied up a killer dispatched by the Kakyuu Group. After letting him go, Banba offers Lin money for information. When Lin agrees, Banba interrogates him and learns that the other man in the photo with the mayor is Zhang. Lin takes the money and leaves.
Later that day, Banba tracks down Lin with the redback spider model listening device he put on him earlier and arrives at the Kakyuu office shortly after Lin kills Ivanov. Then after retrieving Ivanov's cell phone, Banba attaches the listening device to Ivanov's collar. He then leaves with Lin, who, due to his wounds, loses consciousness. Banba takes him back to his office.
Not long after Lin regains consciousness, Shigematsu arrives at Banba's office with information about the man suspected of murdering Qiaomei Lin, Takuya Itou (Saitoh). But having not really learned anything new and agreeing that Itou had nothing to do with it, Banba accepts Lin's request to locate Qiaomei's killer and asks for five years worth of mentaiko as payment.
Enokida arrives to share his findings into the killers hired by the mayor. He shows Banba and Lin the footage from the surveillance cameras at the hotel Takuya Itou was staying at, revealing he was set up by Reiko and Munakata. Enokida plays them the recording from the bug Banba planted on Ivanov's body and hear of Yusuke Harada, the likely culprit of Qiaomei's murder. Lin formulates a plan to have himself be sold to Yusuke as a woman trafficked by the Kakyuu Group in order to kill him and asks for Banba's and Enokida's help.
After taking Lin back to his office, Banba pays a visit to Genzo, who is a killer agent. Genzo tells him that a request to kill both him and Lin came in from the Kakyuu Group, who have hired the Niwaka Samurai to carry out the job.
Banba then heads to Saeki's cosmetic clinic, where he makes an offer to buy a body which had just been dropped off by Jiro. Since Jiro is still at the clinic, Banba asks him to avenge Qiaomei's rape and death and lets him know that Yusuke is the culprit. Later that day, Banba, as the Niwaka Samurai, has already been to the Kakyuu Group office to accept his assignment from Zhang.
As planned, Banba meets with Lin, disguised as a Kakyuu Group broker. He gives Lin a redback spider model device and a microphone to call him in case of trouble before having Lin hide in a suitcase. Banba goes to the meeting and the suitcase with Lin inside is transferred to a car trunk.
When Lin is faced with Zhang and realizes he has been set up by Enokida, he calls Banba for help through the microphone. Banba is called by a distressed Lin for backup but ignores it and continues to keep up with his Niwaka Samurai persona. He is ordered by Zhang to behead Lin but instead cuts down Zhang and his men, including Shinohara and Munakata. After he releases Lin from his bounds, he removes his mask and reveals his identity to Lin. Shortly after, Shigematsu arrives to clean up the crime scene and allows Banba and Lin to leave to confront Yusuke.
They go to Yusuke's apartment building where they meet with Jiro and Saitoh. Banba is wearing the Niwaka Samurai mask again. He waits for Reiko to call Munakata's phone, which he had retrieved earlier, and introduces himself as the Niwaka Samurai and announces he is coming for Yusuke. When she rushes out of the door to escape, they capture her and knock her unconscious before Banba and Lin head inside to look for Yusuke. Yusuke comes out of his hiding and ambushes them with a gun, but before he could shoot at them Saitoh throws a baseball at him and incapacitates him. They take Yusuke and prepare to take him to a warehouse to carry out Qiaomei's revenge, but before they leave Banba invites Saitoh to join their grass-lot baseball team, the Hakata Tonkotsu Ramens.
Some time later, Banba and Lin go to a warehouse where Yusuke is kept bound to a chair. Banba interrogates Yusuke while Jiro records his confession, which they send to Enokida later so he could broadcast the confession to overthrow the mayor. Banba and Lin leave before the avengers start torturing Yusuke.
A few days later, Banba asks Lin where he would go now and upon hearing he has no place to go, offers him a place to stay at his office. Around this time, he also convinces Lin to join their baseball team, despite Lin not knowing anything about baseball.
- He comes from Hakata and speaks in the local dialect.
- His favorite food is Hakata tonkotsu ramen and mentaiko (another Hakata specialty).
- He is a huge baseball fan and his favorite team are the local Fukuoka Hawks. He uses their theme Iza Yuke Wakataka Gundan (いざゆけ若鷹軍団) as his ringtone.
- He owns a Mini Cooper described as red with a white line in the light novel.
- It is shown as blue in the first anime trailer but is red in the anime itself.
- He knows little about technology and owns a few years old cellphone instead of a smartphone.