Context: On August 2019, Crypton Future Media decided to switch from using the VOCALOID engine to their own Piapro Studio engine, meaning among other things that Hatsune Miku was no longer a VOCALOID starting from the Hatsune Miku (NT) voicebank onwards. Ever since then, the Piapro engine has been criticized by users and listeners for various faults and deficiencies.
Meanwhile, on October 12, 2022, the next iteration of VOCALOID, VOCALOID6, was announced. And to everyone's surprise GUMI was also revealed alongside that announcement to be the first non-standard V6 voicebank, GUMI (AI Megpoid). She hadn't received any news or updates for years until that point so it felt very out-of-nowhere.
Finally, after five years of no major updates to the Piapro engine, at the start of this month (August 1st, 2024) Crypton announced that they would be returning to VOCALOID for Miku's next voicebank, Hatsune Miku (V6 AI). (Though they also announced a new version of Hatsune Miku NT so they may not have given up on Piapro completely.)
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.
