In your opinion, which hero has the most charisma in the campaign?

Stalin, firstly, is not particularly charismatic, in my opinion, but even if he was, he only intimidated those who held some power. All other Soviet people fought for an idea, not because they were afraid of concentration camps.

Ordinary officials and NKVD officers had no power, but they were among the main targets of Stalin's repressions. Saying "all the others" is an exaggeration. This was a country where half the population lived in collective farms and starved, but dared not even squeak.

The Overlord was originally just a simple demon, not some "overpowered demon lord." He gathered followers around him, staged a coup, eliminating six demon princes, and came to power in Sheol. This is the canonical story. So, he turned out to be a strong leader.

These six princes died in battles, and only Kha-Beleth remained. He became the Overlord, overthrowing Mal-Belet and establishing an absolute monarchy in Sheol. He prevented the demons from descending into civil war, and given the nature of demons, this was quite possible: when six senior chaos princes died, and only the younger ones, who had long been denied power and were even hungry for it, remained.