Yeah I don't know if it's so much Souji himself being FULL OF HOPE as him having the ability to inspire hope in other people. His S. Links are by and large people who are absolutely miserable with themselves that he helps to overcome their difficulties and become better people. Ai is a great example of this, too, if you've ever finished her link, and from as far as I've gotten it, Naoki, too. And even the Devil and Tower links. I still have never gotten very far on Temperance because Eri is kind of an idiot but yeah, you get the point. Souji may not be FULL OF HOPE ... but he kind of is, actually. He doesn't say much, really, but he has a constant drive to do the right thing, to uncover the right answer, and faith that even if it's difficult, he will find that answer. It's brought up pretty early in game, iirc -- I forget whether it comes up in the Velvet Room or the club first, but Shadow Teddie asks you and Igor does as well, if you think you can find the truth even though it's masked in fog, and Souji's answer is pretty much "well, duh."

I think it's fair to say, in addition, that Adachi was a "force for despair" in the same way Souji was a force for hope. It's hard to say Namatame was a "force for emptiness" since a lot of his hollowness is very personal, but I think you could apply it to the bad ending where he's not killed but Adachi isn't caught. There are answers but not enough of them, and you leave town feeling just ... unsatisfied.