Reisen: Wait, Tewi! Stop right now! Tewi's sign: Prank successful! Tewi: I'm not stopping, usa! Reisen: I'm getting mad! Tewi: So what, usa? Text on Reisen's shirt: Carrot
"Usa" is an abbreviation of "usagi" (兎), which means "rabbit" in Japanese. Tewi, being a youkai rabbit, uses this expression as a kind of verbal tic or catchphrase that reflects her rabbit nature. It's similar to how some characters in media have a distinctive speech style that highlights their identity.