The dragon tattoo on his back earned him the nickname "The Dragon of Rock Bottom" (どん底の龍, Donzoko no Ryū).
Described as having been born with "nothing but physical strength," Kasuga has a balanced muscular build that gives him the opportunity to be proficient at a variety of fighting styles. His eyes are grey and he has medium-length dark hair styled into an afro after his release from prison, as well as a beard. Prior to this, his hair was styled into a much shorter punch perm, growing out long enough during his time in prison to be tied into a short ponytail.
Kasuga's tattoo covers most of his back and ends just above his thighs. It depicts a koi dragon (龍魚, ryūgyo), the in-between state of the koi in the legend of the koi and the dragon. It represents the strength and perseverance of the koi as it struggles upstream, but also a greater destiny that lies ahead, as the koi has not fully finished its transformation into a dragon.
- Ryu ga Gotoku Online: Kasuga's inspiration for the tattoo was Jo Sawashiro, whose tattoo depicts the same creature, as the two both hold a strong admiration for Masumi Arakawa and view him as the 'dragon' they aspire to be. Only the outlines of the tattoo are filled in up until Chapter 21, but it is seen in its completed state in the rest of the game.
Kasuga's signature suit consists of a maroon jacket and blazer worn over a white shirt with an upturned collar and a gold chain necklace. When assigned his unique class, Hero, the way he envisions his attire changes slightly in battle; his sleeves are rolled up to the elbow, he wears black leather gloves, and his necklace is replaced by a red silk scarf.
Kasuga is fiercely loyal to his yakuza family and appears to have a great sense of imagination, which is why many events in combat are so extravagant and outrageous.
Kasuga was born to Akane and Masumi Arakawa on New Year's Day, 1977, in Shangri-La. He was hidden in coin locker #100 at Shinjuku Station by Akane on Arakawa's instructions in order to keep him safe while they were being pursued by members of Hikawa Industries; however, Arakawa unknowingly retrieved the baby locked in the adjacent locker #99. Kasuga was instead taken in by Shangri-La's manager, Jiro Kasuga, and raised by the people of Kamurocho. Although beloved, he lived in poverty and the people who raised him were considered "trash" by the rest of society. To escape this reality, he spent his nights playing Dragon Quest in the back office of the soapland and dreaming of being a hero one day.
After the death of his father he stopped going to junior high school at fifteen and resorted to whatever means necessary to earn money and support himself, leaving his education behind for good. He spent most of his days fighting people for money, regardless of who they were. However, one day, he severely injured and hospitalized a yakuza. The family the yakuza belonged to sought revenge and managed to capture him. They beat Kasuga to the brink of death, and fearing for his life he blurted out the name of a feared yakuza, Masumi Arakawa, saying that if they killed him they'd be hearing from him; in reality, he had never met the man, and was only aware of him due to his reputation as "Killer" Arakawa.
Kasuga hoped mentioning his name would somehow get him free, but it only backfired. The family that held him captive were enemies of the Arakawa Family. They summoned Arakawa to their office and asked him to collect Kasuga in person, assuming Kasuga was part of his family. To Kasuga's surprise, Arakawa arrived. The men demanded he take responsibility for Kasuga's actions. Arakawa offered them his finger in exchange for Kasuga's freedom and the men quickly let Kasuga go.
Kasuga followed Arakawa and tried to thank him, but Arakawa punched him, saying he didn't cut his finger off for his sake. He explained to Kasuga that being a yakuza meant you had a reputation to uphold, and he would have lost face if he simply bowed his head and asked for him back. Kasuga asked him why he would go to such lengths for someone he didn't even know, to which Arakawa responded, "Maybe I just wanted to look cool in front of a kid who knew who I was."
Kasuga decided then and there he'd follow Arakawa for the rest of his life. He waited outside of the Arakawa Family office for one hundred days before Arakawa finally broke down and agreed to swear him into the family at sixteen, against Jo Sawashiro's wishes. The two went on to become akin to father and son, with Kasuga being tasked as his adoptive brother, Masato Arakawa's carer some time after his induction to the family.
Yakuza: Like a Dragon: 2000-2019
On the 31st of December, 2000, Kasuga is called in to the Arakawa Family office, where Arakawa informs him of Jo Sawashiro's involvement in the murder of a member of the Sakaki Family. Arakawa asks Kasuga to shoulder the blame and go to prison on Sawashiro's behalf. Kasuga is more than happy to oblige, believing this to be the chance he had been waiting for to repay Arakawa for all he'd done for him. On January 1, 2001, he turns himself in to the police.
In 2019, Kasuga is freed from prison, returning home to Kamurocho. He is surprised and angered to find that while he was away, the Tojo Clan was exiled, and the Omi Alliance is now in control. He is even more shocked to find out that his boss and father figure, Arakawa, was supposedly the man who betrayed the Tojo and let the Omi into Kamurocho.
- As Nick Ogata explains, the name "Ichiban" (一番) literally means "number one" in the context of being the best.
- Kasuga shares the same voice actor as Akira Nishikiyama, Kazuhiro Nakaya.