Hakkai Shiba is a prominent character introduced during the Tokyo Manji Gang's conflict with the 10th Generation of the Black Dragon gang, an arc that sets the stage for his development. He holds the position of Vice-Captain of the Second Division of the Tokyo Manji Gang, serving directly under his captain and mentor, Takashi Mitsuya. Physically, Hakkai is a tall teenager with an above-average build, standing out with his royal blue hair styled in a crew cut that features a distinct swirl design shaved into the left side. He has indigo eyes and a noticeable vertical scar on the left side of his lip, and he wears a single concentric teardrop earring in his left ear. Despite his imposing stature, his personality is initially timid, friendly, and somewhat awkward, particularly around women other than his sister.

The core of Hakkai's character is defined by his traumatic family life. He is the younger brother of Taiju Shiba, the fiercely powerful and brutal leader of the Black Dragon, and Yuzuha Shiba, his older sister. In the absence of their parents, Taiju dominated the household through extreme physical and verbal abuse, framing his violence as a twisted form of love and discipline. To protect Hakkai from the worst of their brother's beatings, Yuzuha volunteered to take the punishments in his place, a sacrifice that filled Hakkai with profound guilt and an inability to stand up for himself. This dynamic left him deeply scarred and fearful of his older brother, whom he views as a monster.

Hakkai's primary motivation throughout the Black Dragon conflict is to protect his sister and finally free his family from Taiju's tyranny, even if it means resorting to desperate measures. Believing there is no other way out, he resolves to kill his own brother, a plan that his friend Takemichi Hanagaki is determined to stop in order to save Hakkai's future. His role in the story centers on this internal and external battle. Key relationships heavily influence his path. His devotion to Takashi Mitsuya is immense; he sees Mitsuya not only as a captain but as an older brother figure who taught him respect and the importance of using one's strength to protect others. Hakkai's admiration for Mitsuya is so great that he keeps his captain's photo as his phone's wallpaper. Takemichi also becomes a crucial figure, as Hakkai feels he owes him a life debt, referring to him with the old-fashioned term "onjin".

Hakkai undergoes significant development during the climactic battle in a church on Christmas Eve. Forced to confront his brother directly, he initially falters, trying and failing to use a knife. However, witnessing the resolve of his friends and finally accepting his own shame for hiding behind Yuzuha, he finds the courage to fight Taiju with his own fists. Though outmatched, he repeatedly gets back up, symbolizing his breaking free from years of psychological abuse. This redemption allows him to shed his timidity and finally stand on his own. In terms of notable abilities, Hakkai is a highly capable fighter and the second-strongest member of the Second Division. His natural athleticism and punching power are such that he can knock out an average delinquent with a single blow, a skill he uses effectively even against far more powerful opponents like his brother Taiju and Ran Haitani of Tenjiku. For the War of the Three Titans Arc, Hakkai's journey of self-assertion continues as he remains a loyal and vital member of Toman, applying the strength he has reclaimed to protect his comrades in the escalating conflicts against new and powerful enemy gangs.