Tadashi Nanakawa is a freshman at Kirameki High School, standing 180 cm tall and featuring light brown hair with blue eyes. Born on February 2nd under the Aquarius sign, he possesses blood type B. As one of the protagonist's closest friends alongside Manabu Kobayashi, his athletic physique and captivating appearance fuel considerable popularity among female students. However, he consistently deflects romantic advances, offering only cryptic responses regarding his disinterest in relationships.

His athleticism shines during the school's Sports Festival, where he excels in events like the 100-meter dash. Tadashi dedicates himself to physical training and body conditioning as primary hobbies. He shares a close bond with his twin sister, Rui Nanakawa, who attends a different academy. Rui occasionally adopts Tadashi's guise to infiltrate Kirameki High.

Within the game "Tokimeki Memorial 4," a secret route conditional on specific actions during Rui's storyline sees Tadashi confess feelings to the protagonist. While briefly reciprocated, this encounter is later revealed as a dream sequence. This scenario remains absent from the series' animated adaptations and non-canonical. Specifically, the OVA "Tokimeki Memorial 4 Original Animation: Hajimari no Finder" omits Tadashi entirely—he receives no mention, dialogue, or visual depiction, leaving him without background or narrative role in this production. His sister Rui appears as a supporting character.

The surname "Nanakawa" translates to "seven rivers," and his given name "Tadashi" combines the characters for "right, proper" (正) and "will, purpose" (志).