Alden was just some old man. Kessler was Cole from the future. Sasha was some crazy bitch. Who was the "nemesis" or the "demon" that Kessler came back in time to destroy? We never got to fight it. All Cole did in the main story mission was work his way up to defeat Kessler. And destroying the Ray Sphere must not have been that big of a deal, because Kessler made that shit himself. And he didn't clear the city from enemies either, that stuff is in the side missions.  
 
So what the hell did Cole do? How did he make a difference? Did he defeat the nemesis? Am I missing something?

Yes. 
 
 
You are missing a sequel.

I thought it was Alden that made the Ray Sphere. And Kessler was trying to stop it...  
 
But yeah, he did mention some one you never fought. That's why he killed Trish, right? To stop Cole from being held back by her or something. 
 
That may be wrong since it's be about 3 months since I've played inFamous.

So basically the whole plot was there to warm us for a franchise. I hate it when they do that!

Cole's Nemesis, no matter how stupid it sounds, is himself. Kessler went back in time to stop himself because he thought he was weak for marrying Trish, and running away, and wouldn't be able to face the dangers to come.

" Cole's Nemesis, no matter how stupid it sounds, is himself. Kessler went back in time to stop himself because he thought he was weak for marrying Trish, and running away, and wouldn't be able to face the dangers to come. "