I wanna know if his dialogue style in japanese matches the one of gaster to theorize
The Japanese dialogue for each chapter is in “DELTARUNE\chapter3_windows\lang\lang_ja.json” as plaintext. He speaks in hiragana and katakana but not kanji, which is typical of the dialogue in the board games. This is also how people talk when in the light world, and how people talk in Undertale. It’s a reference to classic Japanese video games from before computers had font support for a few thousand kanji, so dialogue had to be written this way. Whereas the presumably Gaster intro voice speaks in katakana and kanji, with no hiragana (which is a weird way to write).