Tokio Yamato (known as Tony Goodman in English adaptations) is the fraternal twin brother of Sora Yamato and nephew of Dr. Bernard Goodman, residing in 25th-century Tama City, Japan. He frequently joins the protagonist on time-travel missions. His appearance features brown hair, blue eyes, an orange top, dark green shorts, brown boots, and typically a translator badge on his chest.

Personality-wise, Tokio possesses a future-oriented mindset and aspires to become a Time Detective, though he occasionally doubts his own capabilities. He exhibits cynicism regarding others' usefulness and struggles academically compared to his sister, preferring experiential learning and adventure. A notable vulnerability is his phasmophobia (fear of ghosts), demonstrated in specific incidents. His development includes moments of maturity, such as knowing when to stop arguments with Sora and confronting his fears during missions.

Relationships are central to his character. His bond with twin sister Sora combines protectiveness with resentment over her perceived perfection; a formative childhood incident saw him give her a bug, causing her lifelong entomophobia. He treats the caveman companion Genshi like a younger brother, though with less influence than Sora holds. Interactions with the vampire-descended Time Detective Kyoichiro Narugami involve skepticism about his competence and heritage. Among the Time Shifters, his closest connection is Wing, whom he considers a "brother" after collaborative experiences; this culminates in Tokio appealing to Wing's true nature during a critical series finale moment.

Tokio possesses only ordinary human abilities, lacking exceptional physical or intellectual prowess. His background includes discovering the fossilized Time Shifter Lovelove in the series' first episode. The surname "Yamato" references a historical Japanese province. Animation occasionally shows inconsistencies regarding his translator badge's visibility.