Yup, something is not right with Sigma I think. Dialog is strange whenever one
The plot thickens!
refers to his appearance. "Ha! Now that's funny. You must've done a lot of drugs to look like that at twenty-two," said Phi. And she called him Grandpa too. Sigma in the artwork and stuff looks kind of...nice looking and not bad at all. So maybe he is some Cradle Pharm shareholder or some shit with an edited memory or a GAULEM with the APB skin! I noticed they don't show his face during escapes like Junpei did either.
The plot thickens!