Kihachi Sakamoto is the eccentric, deeply cynical co-protagonist of Sparks of Tomorrow. Driven by a brilliant but locked-away mathematical mind, he co-authored the 20th Century Electrical Catalog as a boy, dreaming of an electric future. However, after his older brother vanished into the war with their blueprints, Kihachi lost faith in his inventions. He spent years living a reclusive, aimless life in a traditional Kyoto boarding house, entirely disillusioned by the smoke-choked, steam-dominated world around him. Cynical, dry-witted, fiercely loyal under pressure, and secretly burdened by intense survivor's guilt over his missing brother

Inako Momokawa is the 15-year-old co-protagonist whose deep personal convictions and desire for freedom ignite the entire plot of Sparks of Tomorrow. As the younger daughter of a traditional, strict sake brewer in Fushimi, she has spent her life being criticized for her clumsy nature. Despite her outward timidness, Inako possesses an unyielding spirit and a profound, unique faith—often speaking directly to a god she envisions in her own way, which gives her the courage to defy societal expectations Earnest, clumsy but resilient, deeply spiritual, compassionate, and fiercely brave when protecting the people she loves

Yosuke Mizoe is the primary antagonist of Sparks of Tomorrow. As the flamboyant, fiercely competent heir to the Mizoe Steam Engine Company, he represents the absolute pinnacle of corporate power in alternate-history Kyoto. Yosuke views the emerging dawn of electricity not as scientific progress, but as a direct existential threat to his family’s lucrative, smog-choking steam monopoly. Brilliant, theatrical, and entirely ruthless, he serves as the perfect ideological and personal foil to Kihachi Sakamoto Arrogant, charismatic, sharply calculating, and possessive. Beneath his eccentric, dramatic exterior lies a cold, pragmatic mastermind who will stop at nothing to protect his family's legacy

Seiroku Sakamoto is the catalyst for the entire plot and the late/missing older brother of Kihachi Sakamoto.he was a brilliant, free-spirited, and highly charismatic inventor passionately devoted to ushering in a new "Age of Electricity". He initially inspired a young Kihachi to dream big, prompting the two to co-author the revolutionary 20th Century Electrical Catalog. However, Seiroku took the blueprint catalog with him when he left for war and never returned, leaving a massive void, unresolved mysteries, and intense survivor's guilt behind. Enthusiastic, free-spirited, and fiercely creative. He loved to playfully tease his younger brother, but he was also protective enough to secretly save their electrical blueprints when their traditional father attempted to destroy them

Noriko Momokawa is Inako’s fiercely protective older sister and a vital supporting character in Sparks of Tomorrow. As the eldest daughter of the Momokawa sake-brewing family, she bears the weight of upholding her family's strict traditional values, yet she harbors a deeply rebellious streak when it comes to safeguarding her sister's freedom. Sharp-witted and physically formidable, Noriko acts as the primary shield for Inako against both their overbearing father and the corporate forces of the Mizoe family. Stoic, fiercely loyal, pragmatic, and maternal. While she can be stern and demanding with Inako to ensure she grows strong, her actions are always driven by profound, unconditional love for her family

Kengo Kuga is a powerhouse supporting character and a vital muscle ally to the protagonists in Sparks of Tomorrow.Kengo is a massive, intensely strong former soldier who is engaged to Inako's older sister, Noriko Momokawa. Because of his overwhelming physical power and unstoppable momentum in battle, he earned the fearsome nickname "Kugajouki" (Kuga Locomotive). Stoic, fiercely loyal, honorable, and disciplined man

Izō Masubuchi is the strictly professional, highly capable secretary and personal bodyguard of the primary antagonist, Yosuke Mizoe.Izō serves as the quiet, dangerous enforcer behind the operations of the Mizoe Steam Engine Company Stoic, fiercely loyal, deeply observant, and highly disciplined. He is a lethal physical threat on the battlefield but functions entirely out of a protective instinct for his childhood charge

Saki Mizoe is a sharp, fiercely independent supporting character in Sparks of Tomorrow. Voiced by Aya Endō in the anime, she is the older sister of the primary antagonist, Yosuke Mizoe. Rather than participating in her family’s massive steam empire, Saki carved out her own path as a highly successful businesswoman dominating the lucrative wine and spirits trade Cheerful, laid-back, highly pragmatic, and completely unfazed by small details

Suzu Harashima and Kate Okura are Inako’s 15-year-old classmates who serve as her vital support system and best friends

Yajiro Yagura is Kihachi's intensely shy cousin and a gifted artist who maps escape routes and copies blueprints to support the group