Aki Kaidou is the second eldest of four brothers in the Kaidou household and the fraternal twin of Shima Kaidou. He stands slightly shorter than Shima with black hair and brown eyes, typically dressed in casual jeans and shirts.

Aki shares the same father, Takashi Kaidou, with Shima. Takashi’s first marriage to Haru’s mother, Haruko, makes Haru their half-brother. Haru moved from Canada to Japan at age eight. When Aki was around thirteen, both parents died in a car accident, leaving Haru as the twins' primary caregiver. Three months prior, Takashi and his wife Ruri formally adopted Ren, making Ren Aki’s youngest adoptive brother. Aki initially reacted with hesitation and rudeness toward Ren, resenting the shift in Haru’s attention and underlying family tensions.

Brash, hot-tempered, and bluntly honest, Aki exhibits the least polished social conduct among the brothers. He displays possessiveness, particularly toward Shima, yet assumes a more mature role when Haru is absent. For years, Aki and Shima blamed Haru for their parents’ deaths as the accident’s sole survivor. This hostility persisted until Ren arrived four to five years later. Aki reacted distantly and antagonistically upon discovering Ren at Haru’s apartment—a response Haru anticipated. Eventually, Aki overcame his resentment and accepted Ren.

Aki’s relationship with Haru blends loyalty with underlying jealousy, especially regarding Haru’s bond with their cousin Natsuo. He deeply cares for Haru, who funded his and Shima’s college tuition through host work. With Shima, Aki shares an inseparable bond despite their contrasting personalities—Shima’s calmness offsets Aki’s volatility. Aki fiercely protects Shima, as seen when he lashed out over Shima’s excessive tutoring of a middle schooler named Ai. Toward Ren, Aki’s initial rejection transformed into active concern: he defended Ren from an intruder, scolded Haru for leaving Ren alone, and fretted when Ren stayed overnight elsewhere. Shima once confided to Ren about valuing a younger brother to advise, reflecting strengthened bonds among the three.

Aki attends university with Shima and works at the family café, White Fang. His twentieth birthday near Halloween falls in late October. His name "Aki" uses kanji for "second" (亜) and "tree" (樹), homophonous with "autumn" (秋)—potentially aligning with Haru ("spring") and Natsuo ("summer") if kanji differed.

Across the series, Aki evolves from a resentful, impulsive teenager into a responsible young adult who actively protects his family, including Ren. This growth is consistent through both anime seasons and the ongoing manga.