The Redwall Character Guide
Personality is, well, a large part of any character. It is the most important part because it's the one thing that you absolutely positively CANNOT forget (you may not want to forget your gender and species, but if you do, you're still better off than if you forgot your personality).
Outgoing, or withdrawn?
The first thing to look at when you're figuring out the personality is to answer one question: Is the character outgoing, or withdrawn and shy? Are they going to be both, depending on their situation?
If you chose outgoing, you have to ask yourself, How outgoing? Are they the center of attention at every gathering? If they are, what are the limits of what they'll do to become the center of attention? Are they going to be considered crazy? A ruffian? A troublemaker? A rabble-rouser? What'll best describe the shyest they'll be?
With any withdrawn character, you're not going to have a whole ton of speech. That, however, isn't going to make any difference, because, as any really shy person knows, a whole lot is going around in your head when you aren't talking. So, if you're making your character withdrawn, be sure to write a bit about their thoughts on things. As with outgoing personalities, figure out the limits. What's the most they'll say? How long will they go without saying anything at all? Will they even go to a gathering?
What is right and wrong?
Here is where you'll have the biggest problem, I think. Determining your character's sense of right and wrong takes a leap of imagination, especially if it's any different from what your ideas of right and wrong are. So, when you're just starting it off, have their ideas about what's good and bad be your ideas. Of course, if it's a vermin, you have a few choices. You can either take your ideas about good and bad then reverse them, or you can just have them not care one whit either way. For good beasts, however, it's hard. The best thing to do, I've found, is to model it off of people you know, like your parents, siblings, or friends. Chances are that you'll find that in some spots they'll differ from you.
Here's another one to be wary of. Are they going to try and get out of any type of work, or are they going to delve in and end up doing the lion's share? The former would make them less well thought of, while the later will make them respected.
Ideas of fun
What will make your character happy? What do other people think about what makes them happy? What will make them sad? What will make them angry? What will make them triumphant? Think about it all, and think about how they're willing to show it. It's one thing to say, "So-and-so was angry", which will say it, but it would be better to say, "So-and-so's eyes scrunched together as he bared his teeth and snarled a ferocious war cry"