Nemesis Prime is an evil clone of Optimus Prime, or something like that. I don't really know, but the idea of an evil clone is cool, so that's what I think of him as. For more info about this character, see the [Nemesis Prime article on TFWiki.net]
Nemesis Prime is a repaint of Classics Optimus Prime, and was released as the San Diego Comic Con exclusive figure in 2008. I, of course, wasn't collecting Transformers at the time, and so had to pick this one up on eBay in 2010, however I got him for a good price, so I'm not complaining.
As with Classics Prime, this is an excellent mold, with a nice, modern look to the cab-over-engine type truck, and a very iconic looking bot mode. I love this mold, and am trying to acquire all repaints of it, but a couple of them are ridiculously expensive (Shattered Glass Prime, Wonderfest Black Convoy, Crystal Convoy, which fetch many hundreds on the after market) so I'll either have to get knock offs or go without.