Tina Ieri, originally named Koyomi Himekuri, partners with Beppe Domani (Wataru Toki) as a central character. They live in an apartment building managed by Mr. Arsenio Maigret, assisting the investigator while navigating their own dynamic. Though her family background isn't extensively detailed, Tina is revealed as a direct ancestor of Princess Domenica from 2980, linking her lineage to the royal succession of the Calendar Kingdom.

Since childhood, Tina possesses a lock-shaped talisman keepsake that pairs with Beppe's key-shaped talisman. Uniting these items summons the robot King Star, essential for confronting adversaries during their time-travel missions. Clever and spirited, she often displays assertiveness and impatience with Beppe's perceived cowardice, remaining unaware for much of the series that he transforms into the superhero Yattodetaman – a central narrative tension.

Her relationship with Beppe evolves significantly. Initially dismissive of his timid nature, she develops romantic feelings for his heroic alter ego, Yattodetaman, without recognizing his identity, creating ongoing dramatic irony as she contrasts admiration for the hero with frustration toward Beppe. Tina actively participates in missions, traveling through time to locate the mystical Cosmopeacock crucial for Princess Domenica's rightful claim to Calendar's throne. She engages directly in battles, including hand-to-hand combat with antagonists like Ottobre before King Star's summoning.

Her character arc resolves when she discovers Beppe's dual identity as Yattodetaman. This reconciles her feelings for both personas, resolving the romantic subplot as she accepts their shared future of marriage and descendants in the Calendar Kingdom.