#other theories of mine include that fukuzawa wasnt an assassin and that the tripartite framework

[kaurwreck]

I'm certain now that bsd!Sigma is Yukio Mishima. (Although, that his character design parallels Shibusawa's was reason enough.)

In math, sigma is the symbol Σ, which indicates the summation of numbers. The kanji in Yukio Mishima's (三島 由紀夫) name translates literally to three islands + cause/reason for, chronicle/record, man.

Sigma, who has only three years of memories, chose his reason for existing to be the sky casino, a floating island. He's also lonely, an island unto himself, with only three years of memories. In other words, Sigma is the base sum of his parts.

(Of course, Sigma wasn't born three years ago because, aside from being nonsense, Dazai can touch him, and Dazai hinted at the absurdity when he teased Sigma. Never mind that it doesn't make sense with reality narratively, thus violating the "rules" of the Page, and that Fyodor has performed lobotomies before, with Ivan.)