Sera Naoki is the junior of The Big Four's Nakajima Shinsuke and currently the Head of Kawada 2nd High. He is a childhood friend of Suzuran's Mutou Renji and brother-in-arms of Yuri-South's Head, Abo. He was first introduced as a witness to the 5 vs 5 fight between Housen and Suzuran. Highly touted as a strong fighter, Sera is the only man other than Tsukishima Hana to remain standing after a fight with Sakota Takefumi in middle school.
Because he lives on the outskirts of the Town, Sera and the rest of the students at Kawa 2nd usually take the brunt of the first wave of any attack the Town is under. But this is perfect territory for him, as Sera is the Head of the strongest school in his area and, as such, commands the respect of many of the Town's key players. He is able to hide his calculating and intense personality well beneath a carefree, devil-may-care attitude. However, it's this same attitude that also gets him into so much trouble.
He initially remained winless in the main storyline, despite his reputation,until defeating the Kokumon Group's Itami Ryousuke. During the war against the Amachi Army, Kawa 2nd was the last school to fall before Housen and Suzuran. Initially mistaking Muroto Koumei as his opponent, Sera makes a grave miscalculation and ends up facing-off against the Army boss himself, Hisashi Amachi. While he had the upper hand at first thanks to quick wits, Sera suffers a solid defeat at Amachi's hands, thus completing the Army's rapid ascent to power in their area.