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.