Akihiko Kaji stands as a central figure, initially appearing as a college student and drummer. His sharp, angular features include piercings on his left lip, tongue, and ears, short spiky blond hair, and a tall, athletic build. Early depictions present a rugged exterior reflecting inner turmoil, though his demeanor later softens, showing frequent smiling and playful expressions.

Born to divorced, neglectful parents, his background involved significant instability. He pursued violin competitively in high school, achieving national recognition. Enrolling at Keio University as a violin major, he grew disillusioned with classical music due to emotional struggles stemming from his toxic relationship with Ugetsu Murata. This led him to explore other instruments, becoming proficient in drums, guitar, and bass, with musical tastes spanning classical, metal, and experimental genres.

Personality-wise, he presents surface confidence and aloofness while harboring deep-seated abandonment issues and self-destructive tendencies, including chain-smoking and patterns of unhealthy relationships. Perceptive yet emotionally guarded, he struggles to articulate feelings despite intense passion, especially regarding music and intimate connections. Contradictions define him—charismatic yet vulnerable, carefree yet burdened by unresolved pain. His broken family amplifies fears of abandonment, causing him to view bandmates as irreplaceable support systems.

Key relationships drive his development. His co-dependent, on-again-off-again dynamic with Ugetsu involved mutual idolization and emotional enmeshment, measuring each other against unrealistic standards. Toxicity culminated in Akihiko moving out after violent confrontations, symbolizing initial steps toward self-reclamation. Subsequently, his bond with bandmate Haruki Nakayama evolved from initially "triangling" Haruki for stability during his breakup into genuine romantic attachment. His growth included learning healthy commitment—shifting from possessiveness, like resenting Haruki's support for other bands, to supportive partnership, marked by gestures like wearing matching aesthetic rings and prioritizing Haruki's comfort. Over a decade, he cultivated vulnerability, expressing jealousy more constructively and striving to become "a man deserving of Haruki's love."

He also formed a mentor-like connection with Mafuyu Sato, recognizing their shared experiences of familial neglect and abandonment trauma. He advised Mafuyu during identity struggles, reflecting his role as an empathetic foundational figure.

Musically, he serves as the band's rhythmic anchor. Though disillusionment drove him from violin to drums, he later returned to competitive violin, placing fourth in a university competition. This reintegration of classical and contemporary influences signified emotional reconciliation. His multifaceted skills contribute to the band's creative tension, and he encourages members to push artistic boundaries, like insisting on impactful performances to impress new listeners.

Long-term development shows significant change. Early self-loathing and avoidance behaviors, such as casual sexual arrangements during homelessness, gave way to accountability and stability-seeking. By the "10th Mix" timeline a decade later, he exhibits consistent softness, using playful disguises and prioritizing Haruki's happiness. Though traces of insecurity persist, like discomfort with Haruki interacting with exes, he consciously channels these into healthier communication.