And apparently it isn't a Transformers character - at least in the Movie.
As with previous attempts to have certain cars as Decepticons, apparently MB blocked the usage of their vehicle as 'evil'.

But I don't see how a car company, desperate for media exposure during these tougher times in the auto industry, could prevent Bay from doing anything he wanted. He'd just simply say to them, 'This car will be a Decepticon, or else I'll get a different car company to play the role to millions of viewers'.