That is a good question. Hmmm... As they are now.
As they are now? Sure. I'd say something like...
Dave - Paladin. Wants to do the 'right' thing and is considerably "Lawful Stupid"
Tyson - Rogue. Maybe Barbarian if only because there's not much going on upstairs but a whole lot of anger to work through, but stealing is the tipping point.
Roswell - Black Mage/Warlock. Any occult-oriented mage will do. Reasons should be obvious. Any high INT based class works well here.
Orlando - Healer. One of his costumes in beta (and reworked into the new sprites) hints towards this. He's pretty group mom anyway.
Dean - Druid. Nature boy through and through, but pretty resourceful beyond that.
Hoss - Bard. Reasons here should be obvious.
Sal - Fighter? He's hard to place given his physical abilities and personality sort of clash. But he's another protector-type role.
And of course, all this is how they stand now. How they shift post-story is more interesting but we're not at that point just yet for it to make sense :P