Sunetsugu is Suneo's younger brother, living in New York with his uncle—a single guy who basically raises him as his own kid. He's around seven or eight years old, though why he's there instead of with the rest of his family remains unclear.
Early on, Sunetsugu came across as pretty arrogant, especially toward Nobita, which honestly made him feel like a mini version of Suneo. But his character shifted over time, becoming more polite and genuinely naive, which fans tend to read as a redemption arc of sorts.
Design-wise, he's gone through some changes. In the 2005 episode "Arigataya," he had brown hair and smaller eyes that didn't really scream "Honekawa family member." Clearly the creators weren't happy with how that looked, because he got a redesign that made him look way more like his relatives—especially Suneo.
Content compiled by AnimeList.moe from publicly available sources.
