Akatokuro brought it up but I think one of the reasons that Sigma is less popular than let’s say, Junpei is, (besides how gross and discomforting he can be) is because Sigma provides the player with a lot less agency.

70% of Junpei’s decisions were motivated by his desire to white knight Akane and provide himself with an ego boost, but other than that he’s essentially a blank slate in regards to morality. You can project yourself onto him much easier for that reason, and the decisions he makes can align with your own objectives

But Sigma isn’t like that. Sigma has a very rigid moral code and emotional perception and he feels things very very intensely, so even if you make him betray others he’ll outright say things like “Why did I do that?” “My hand must’ve slipped, I wouldn’t…” “I don’t understand why I did that!” and then proceeds to become upset upon seeing how the other party’s been affected

Because he didn’t do that. You did.

Even most of the endings in which he picks betray aren’t canon (aren’t necessary for you to get the true ending = not canon since that’s the actual timeline)

Sigma is a failure of an alpha male placed into the position of one, he’s a terrible leader, he’s self sacrificial to an ultimately harmful fault, his moral code damns him and others every time and he feels things to such a strong extent that he has to note it in his own narrative

He’s not a character you can easily project yourself onto the way Junpei is