Mamiko Noto
Chiba enjoyed Shōwa period mystery writers such as Seishi Yokomizo and Rampo Edogawa, and wanted to create a game using that style. The key gameplay and story theme for the title was rewriting the past. The setting's aesthetics were based on Japan in the 1930s and 40s with a fantastical twist. This decision was cemented while Nakaaki was designing the main cast. Referencing his inspiration, Chiba left homages to classic detective story writers in the game, explicitly naming antagonists Dixon and Christie respectively after John Dickson Carr and Agatha Christie. A notable element for "Sigma Harmonics" considering its platform was the extensive use of voice acting for its cast. The cast included Daisuke Ono (Sigma), Aya Hirano (Neon), Tetsu Inada (Dixon), Mamiko Noto (Christie), and Jun Fukuyama (Rin Yukiha). Hirano in particular had to voice Neon in a variety of ways due to the job changes impacting her personality.