All the main and secondary/supporting characters are in it. The only ones who aren't are the majorly minor ones (Egypt, Belgium, and Netherlands, the Nordics for example). Other than that... yeah, pretty much all the characters are in it, but don't expect them to have much screen time.
__I must say, 2/5 Nordics actually did show up. Iceland in the beginning/after the credits, and Finland is shown in his Santa suit waiting for Christmas to come when he's transformed by the Pict.