Thank you for your response! So Rigel Theatre belongs in Grand Thaw? Since there are some songs that have been associated with both Grand Thaw and Rigel Theatre at the same time i.e Aventyr. Your linking to the website is very helpful, never did I thought there was one. But I'm sure that InoueDelta is Rigel Theatre with a different name, isn't it? In the Biography of the website, it said Rigel Theatre's real name is Kouichi _Inoue,_ which pretty much speaks for itself.
From another related videos comment: InoueDelta (井上Δ) seems to be the previous alias of Rigël Theatre. Can confirm this if you go to her website ⇒ Shop ⇒ bandcamp ⇒ Little Red Train. Also, her real name is Koichi Inoue according to the biography on aforementioned site. I wonder why that nick... 🤔