When it comes to console role playing games, the characters play a big part in whether it's actually good and fun to play. Think about it this way: you're going to be spending many hours with these people. Do you really want to play 40, 60, or 80 hours of a game as characters that annoy you? Most would say 'no.'
Tales of Vesperia has a lot of characters. Some are integral to the plot while others are just there for funny story bits. Overall, there are seven characters in Tales of Vesperia that you'll need to pay attention to. These are the characters that will be in your battle party. Some will come later in the game, but all are important to the plot.