Sousuke is a five-year-old boy born on May 25, 2003. He lives with his mother, Lisa, in a cliffside house overlooking the sea. His father, Koichi, is an often-absent ship captain. Sousuke displays notable intelligence and maturity for his age, coupled with loyalty, protectiveness, kindness, and adaptability. He forms deep emotional bonds, most profoundly with Ponyo, a magical goldfish-like creature he discovers trapped in a glass jar on the shore. Freeing Ponyo by breaking the jar cuts Sousuke's thumb. Ponyo licks the wound, healing it instantly. He names her Ponyo and vows to protect her, forging an immediate, powerful attachment. This protective instinct drives his actions. He takes Ponyo to his nursery school, concealing her in a bucket, and later introduces her to residents at the retirement home where Lisa works. When Ponyo's father, Fujimoto, forcibly takes her back, Sousuke experiences deep distress, revealing his emotional vulnerability beneath his resilience. Sousuke addresses both parents by their first names, Lisa and Koichi. His relationship with Lisa is close and collaborative; she trusts him to act independently, such as caring for Ponyo during a storm. His interactions with Koichi reflect admiration mixed with disappointment over his father's absences, though Sousuke maintains contact through gestures like Morse code signaling. After Ponyo returns transformed into a human girl, Sousuke joyfully reunites with her. When a catastrophic flood isolates their home, he takes initiative to find Lisa, navigating the submerged landscape in a toy boat enlarged by Ponyo's magic. He exhibits resourcefulness and determination, pushing the boat ashore when her magic wanes. Fujimoto confronts them, warning of nature's imbalance; Sousuke attempts to flee with Ponyo but is captured. Sousuke's character arc culminates when Ponyo's mother, Granmamare, presents a trial: he must prove unconditional love for Ponyo regardless of her form—human or fish. Sousuke affirms he loves "all the Ponyos," demonstrating unwavering acceptance. Passing this test allows Ponyo to become fully human, restoring natural balance. Sousuke solidifies his protective promise by completing her transformation with a kiss. Sousuke has dark eyes, tan skin, and dark hair with gray roots styled in a bowl cut. He typically wears a short-sleeved yellow shirt and dark grey shorts, often barefoot or in uwabaki slippers. His appearance influences Ponyo's human form. Sousuke maintains strained interactions with characters like his classmate Kumiko, whom he avoids and dislikes. His primary relationships center on his family and Ponyo, with his actions consistently reflecting dedication to those he loves.