Kenichi was born to Mototsugu Shirahama, a businessman to an unknown firm and Saori Shirahama, a homemaker and later in his adolesence, brother to his sister Honoka Shirahama. Contrary to many characters in the series, Kenichi grew up in an average lifestyle and was friends with Ryūto Asamiya, and were best friend back when they were 6 years old mainly because they both were picked on by others, Kenichi's life changed abruptly when he and Ryuto saw a young Miu defend an elder woman who was being harrassed by thugs. Miu was about to be attacked from behind by one of the thugs when Kenichi used Ryuto's whistle to alarm her and allowed her to defeat him and alert her grandfather. However, the thug accidently let a bottle he held fly in the air and bash Kenichi in the head, knocking him uncousious and Miu thanked Ryuto thinking he thinking that he was the one who blew the whistle. After seeing Miu defeat the thug trying to assault a kind old lady, both Ryuuto and Kenichi vowed to become strong, but Kenichi forgot the fact and that it was Miu who traded him the Yin Yang badge he carries on the neck of his shirt all the time for a cat badge, as a remainder of said promise. Because of this, Ryuto would want the badge, but Kenichi would refuse because of Miu giving it to him and the two would fight over it and though Kenichi won, yet he gave the win to Ryuto, leaving Ryuto angry. Kenichi would eventually move one day with his family and not be able to say goodbye to Ryuto due to his father being overly dramatic. This would cause Ryuto's friendship with Kenichi to be destroyed.
Kenichi is good-natured, timid, polite and cowardly, frequently treated as a friendless loser at the beginning of the series. He loves gardening and reading How to... books. However, it all changed when he met Miu Fūrinji, and shows no fear in the face of true danger. After the fateful meeting, he decided to stand for himself and be truly strong; or at least, strong enough to protect those who are dear to him. Though he's constantly considered cowardly at the beginning of the series, he shows that when his loved ones lives are on the line, he's actually much braver than he appears, going as far as to jump into the ocean full of sharks and fight one of them just to save his sister Honoka. He continuously grows much more braver and confident in himself to the point he now rushes into any form of danger without a second thought.
After entering Ryōzanpaku he has slowly begun to change. Although at first he ran at the very first sign of danger, and didn't want to train unless forced by his masters, he endured the hard training and is slowly growing to love martial arts. He was not seen as a serious fighter by any of his opponents, which has led to many victories over seemingly impossible odds (Boris notes that Kenichi has had the bad, or good depending on how one see's it, luck of fighting martial artists who were stronger than him in terms of pure skill in every one of his main battles), and his care for his newly found friends has made him prove victorious time and time again, thus changing his attitude to the point where many of his classmates think he is cool.
Due to Kensei Ma's influence on him, Kenichi has shown a bit of a perverted side, such as when Ma takes Kenichi to a secret location to watch Mui and Shigure bathe in their usual spot (and later uses him as a decoy to escape). Kenichi was even given magazines based off of pictures of Mui and Shigure or other nude pictures (though Kenichi states they are really only pictures of chickens or baby chicks and at times Tochumaru posing covering them). Kenichi even used Ma's technique when he told him to let Miu attack him so his face would be shoved into her chest (which was later proven so Kenichi would learn a headbutt move) and even said how wonderful it felt. However, despite Ma's influence, Kenichi has shown a modest and innocent side towards women, such as when he ever sees Mui's breasts almost sticking out, seeing her almost naked, accidentally seeing up her skirt or seeing Renka's breasts causing Kenichi to be very flustered. When Shigure wanted to bathe with him, he was very flustered and wished not to, showing innocence. Whenever he sees women barely dressed or almost naked, he tends to have a nosebleed.
Unlike most of the other characters, Kenichi tends to think logically, often crushing the dreams of young fighters who strive to create a strong gang in the future. He also sticks to his convictions, be it fair (such as never hitting a guy with glasses) to very controversial, such as his belief to never hit a girl, no matter the situation. This vow often backfires as when women try to fight him, as they become extremely irritated and mistake it for overbearing sexism (this is proven false as Kenichi has no problem with other men, such as Hayato or Natsu, fighting females). Kenichi even cares about the enemy's health and always tries to help them. Despite his strong moral standards, Kenichi is not above buying perverted pictures of Miu and Shigure from Ma or going with him to spy on them while they're bathing. He is still scared of delinquents as shown at his school, despite defeating opponents like Sho Kano and other Yomi members.
Though his masters claim that he has no talent or whatsoever for Martial Arts, this could possibly be a ploy to motivate him to work harder. Despite the fact that Kenichi has not even been studying martial arts for a year, he has defeated countless opponents who have been studying for their entire lives and considered true genius. Hayato Fūrinji, the Elder of Ryōzanpaku has stated that Kenichi reminds him of himself in younger days. The claim could be an encouragement from his masters not to be laid back. While his masters always states that Kenichi has no talent at martial arts, Akisame stated that Kenichi is a genius at hard work. This is further reinforced where Odin commented that Kenichi has mastered the essentials of the Seikūken in just a matter of days.
Though he is no longer picked on by the delinquents at his school, Kenichi's excessive mistreatment by others is one of the main sources of humor in the series. He is constantly ridiculed and looked down on, physically abused for no reason, denied his only pleasures of reading and gardening, and receives no praise at all for his accomplishments and actually only gets ridiculed more (sometimes by people much weaker than himself). As a result of this, Kenichi is frequently shown to be very susceptible to negative comments about his appearance or strength, easily falling into (comical) depression whenever someone scoffs at his abilities (so much so that Miu once referred to name calling as Kenichi's "Weak Spot").
Kenichi seeks "enough power to protect those that he loves". However, the Elder told Kenichi that saying that is the same as stating "I want the power to kill those who are my enemies". What Kenichi really seeks is the power to definitely protect his loved ones by defeating his enemies without causing more than enough damage to and protecting them as well. Hayato Fūrinji stated that this is already the level of strongest in history. Kenichi seems to have another power within him, which his opponents often describe as light in the middle of darkness. This often results in many of those he has defeated turning over to the good side. Rachel Stanley described him as a ceramic vase being beautifully crafted. Isshinsai Ogata also states that Kenichi possesses infinite potential inside him and that his bases are so solid, he could become the highest building ever. On occasions, he breaks down into Dou like combat when someone insults his masters or hurts Miu.
Kenichi has constantly shown to be a very kind, selfless, polite, and helpful young man and always put others before himself. Even when it comes to his enemies, he shows them respect and never speaks ill of them and even cares for their well-being and refuses to allow them to ever speak ill of others. Whenever he's injured or someone else is, he states the other people injured should be tended to first. He constantly tries to befriend others such a Chikage, even if they were former enemies and always acts friendly towards them. It is because of his kind nature that he lacks killing intent due to his desire not to kill others. Several people are moved by his kindness and selfless nature, to the point they even change sides and befriend him. Because of his kindness, Ethan Stanley states that his heart is enourmous. Thor would state his heart has become even stronger lately from his continuous fighting.
Despite his friendly, kind nature, he does enjoy teasing Natsu Tanimoto. He usually does this by annoying Tanimoto (by adding the chan suffix in his name or acting friendly), and once Tanimoto gets so irked to the point that he attacks Kenichi, Kenichi points out people are watching, causing Tanimoto to revert to his false persona.
He has a habit of constantly watering plants and tending to his books when he gets excited. His habit of this kick in when he and Miu held hands. He also is very intelligent in plant types, such as knowing how they work or grow and considers them some of his closest friends, which was shown when he relied on plants to save him from Sho Kanou. He also is afraid of weapons, as he usually shouts "weapons are scary!", though he slowly loses his fear for them. He also has a fear of riding scary or dangerous rides such as rollercoasters, much like Thor (though his fear is mostly due to his hellish training from Akisame's brutal machines which seem to have left a bad impression on Kenichi).
Kenichi is known for being somewhat attractive by numerous girls his age throughout the series due to his strength, bravery, and kind heart. Yūka Izumi loves him due to his kindness and tries to ask him out more than once only to be interrupted or Kenichi leaving before she notices he left and Kenichi is oblivious of this. Li Raichi is attracted to him due to him standing up to her and considering her a friend and admiring him for his kindness. Rachel has shown to find him "cute" and has an attraction towards him, going as far as to kiss him on the cheek, causing Miu to be very angry and jealous from her. Also Rachel also appreciates him and his friends saving her from Jenazad and helping her brother. Renka Ma is attracted to him due to how strong he has become and brave, going as far as transferring schools to be close to him and always trying to cling to him and fight over his attention with Mui and her "strong liking" has eventually developed into "strong love" which she refuses to admit, but her emotions prove otherwise. Also, Renka also is taken back by him when he complements her in anything and blushes around him when he does. Shigure seems to be fond of Kenichi more than just as a disciple, such as her attitude having changed since meeting him. Miu even noted the change in her behavior, suggesting Shigure may have feelings for him, such as smiling for the first time when Shigure commented to him about his relationship with Miu. Miu developed feelings towards him when he proves his worth as a fighter and when she sees how much he cares for her in numerous moments and is very appreciative of everything he does for her as a friend. Miu even gets jealous and possessive of Kenichi when other girls are around him and fights for his attention. When Kenichi saves her life on the School Ski Trip, Miu tearfully hugs him and thanks him for saving her, making Miu much more open physically and emotionally. Whenever Kenichi is done fighting a long fight, she is usually running up to him and tearfully hugging him and states how glad she is when he's alright. When Kenichi tries to state how he wants to protect her more than anything, Miu starts to understand her feelings for him, going as far as kissing him on the cheek. Since then she's started acting more happy, cheerful, and eager to be around him. According to Junazard, Miu's feelings for Kenichi has developed into love (though Miu doesn't seem to be aware of this). Miu has even stated she does not mind dying for Kenichi so he can live. Kenichi has earned the trust of the One Shadow, Miu's father for protecting Miu from Junazard and stopping him and breaking the his spell on her.
- Mixed Martial Artist: Since entering the Ryozanpaku dojo, Kenichi has been trained in six different styles of martial arts: Karate, Muay Thai, Chinese Kempo, Ju-Jitsu, along with learning to defend against weapon users and some techniques from the Fūriunji's personal style. However, his masters have him focus his training on the basic combat forms and techniques, in order to give Kenichi a solid foundation on the martial arts basics and for him to develop his own techniques and fighting style. As a result of practicing so many fighting styles and the hellish training he undergoes on a daily basis, Kenichi developed from a weak person who was repeatedly bullied and beaten to a powerful and extremely versatile fighter, who has received praise and respect on multiple occasions from several disciples and masters, either friend or foe alike.
- Enhanced Speed: He can outrun a truck on full speed due to the amount of running he does with Akisame. Currently, his speed has increased enough to the point he can close a considerable distance in seconds. Although Kenichi has become very fast, he isn't fast enough to keep up with Miu unless she holds back (a lot). His speed has reached a point that during sparring matches with his Akisame, Kenichi can keep up and blocked his attacks (albeit the said master was still holding back considerably). Even in a severely battered state at the hands of Ogata Isshintai, Kenichi still managed to save Miu from a collapsing building on fire, closing a good distance in mere moments.
- Immense Endurance: Even before starting his training, Kenichi possesses a level of endurance that is nearly at master level. Since beginning his rigorous and grueling training, his endurance has grown considerably higher. Even when he was battered, bleeding and barely conscious, he was able to unleash a powerful "Mubyoushi" and "Korui Nuki" against Pengulu. According to himself, he was a prodigy at getting beaten up. During his early fights, Kenichi was able to withstand a body battered and even fractured or broken bones from skilled enemies who trained far longer than him, and ultimately win. Even taking a critical knife wound from Jihan, Kenichi was able to win the battle despite the blood loss. Against Sho Kano, the former leader of YOMI, Kenichi was also able to win despite sustaining broken bones and bruised organs. Kenichi can also take high-voltage electric shocks with little damage (thanks to Akisame's training). Kenichi's endurance now can also take a serious direct kick from Apachai, and quickly jump back to his feet, seemingly unfazed and amazing his masters. The pinnacle of his endurance was seen against another Grand Master, Isshinai Ogata. Despite being hopelessly outmatched and taking a severe one-sided beating, he still had the fortitude to fight on, something Ogata praised him for. Also, after taking a critical blow from Ogata that initially left him incapacitated, Kenichi was soon after able to return to his feet to save Miu from falling rubble.
- Enhanced Strength: While initially Kenichi was scrawny and his strength was below average, the amount of training Ryõzanpaku placed on him greatly increased his overall abilities. His newfound strength has allowed him lift and pull large amounts of weight during daily chores (which the masters force him to do) and knock out less skilled opponents with a single strike. His leg strength has grown great enough to withstand tremendous amount of impact, he was able to kick Genson in a pool without facing any resistance from the water. Having undergone the unique physical conditioning regimen of Akisame, Kenichi's musculature has been rebuilt, now possessing a lean but well-defined physique. He can push Berserker back and throw him at a considerable distance and smash the ground with just a punch.
- Enhanced Agility: During his second training with Hayato, Kenichi's leg strength and dexterity were pushed to higher levels, granting him greater agility and sense of balance. His newfound agility has been commented by the other masters, even forcing them to put a bit more effort in their sparring sessions. Kenichi has also developed acrobatic abilities almost at Miu's level, able to easily scale and glide down a skyscraper with complete control and no wasted movement.
- Immense Determination: The Ryõzanpaku masters mention that Kenichi has no talent for martial art but he is a genius at hard work and getting beat up, which enables him to improve at an amazing rate. Kenichi has defeated countless opponents who have been studying martial arts their entire lives and were considered true geniuses. Even in the midst of battle, Kenichi is able to calculate and develop a stategy quickly and even outsmart even the most intelligent of fighters, such as being able to trick Ryuto's inner eye technique with his Ryozanpaku's Rhythm he just made up, something Akisame noted was a genius move on Kenichi's part. The Elder stated that though Kenichi lacks killing intent, he has incredible memory, being able to remember and percieve even the most complicated techniques his masters teach him and be able to use them in a short amount of time.
- Untapped Athletic Potential: It can be said that Kenichi has athleticism on an elite level largely due to Ryõzanpaku's training. However, the fact that Kenichi has survived the training and reached a great level despite only having trained for a little less than two years in Ryõzanpaku is proof that he has a lot of hidden athletic talent. However, due to his timid nature and relatively peaceful attitude, Kenichi never trained while he was younger. Presently, Kenichi is athletic enough to hold his own against and defeat young martial artists who have trained their whole lives and had their athletic abilities noticed at very young ages. Examples of this include being able to spar evenly with Takeda Ikki, who was a high level boxer even before receiving Master Level Training from James Shiba, Kenichi was also able to finish right behind Boris Ivanov in a swim race, which is deemed impressive since Boris is a high level soldier with outstanding physical condition. One of the best examples is beating Sho Kano, who had the greatest physical potential of all the YOMI members and was chosen by the Kuremisago, an organization specializing in producing gifted young warriors dating back many years.
- Lack of Killer Intent: Despite his strength, it is noted by his masters that Kenichi lacks any form of killing intent even towards his enemies. It is most likely because of this many stronger opponents tend to look down on him and become overconfident in their own abilities, which leads to their downfall and the fact Kenichi's heart is very gentle and his desire to end fights peacefully and only leave them unable to fight any further. Also because of his lack of killing intent, he is able to avoid detection from Master Class fighters for a period of time.
- Immense Sei Ki: Kenichi uses Sei Ki, a style which allows him to fight with a calm and peaceful spirit instead of with rage and anger. This allows him to calmly observe his opponents and see what is in their heart. Even when his emotions get the better of him, he is able to take advantage of his anger and channel it into his Sei Ki, something Akisame noted was a very talented move. On one occasion he instinctively unleashed a strong wave of ki that surprised his masters. Even a master-class like Christopher Eclair was startled by the intensity of Kenichi's ki, momentarily thinking it was Sakaki's. Aside from it's potency, Kenichi's ki control is well developed, allowing him to use highly advanced and complicated Sei ki techniques such as the Ryuusei Seikūken instantly and to a near-perfect degree.
- Immense Tenacity: His greatest attribute was not his power nor his technique, but his tenacity he still maintained his cool and fought putting his life on the line numerous times just to land a decisive hit on his opponent. He has the most important attribute to becoming a martial artist and given praise from a member of the One Shadow Nine Fist Ogata Isshinsal.