It seems you already have the plot down, so I will not mess with that.
As far as character creation goes, the conceptual art seems to show the character traits of the warrior characters pretty well. Strider could use some more work, but if you INTENTION is to portray him as a normal cat, then you are right on target. Many people try to add character to their characters by making them look unique, or give them some special powers, but real characters have a soul. Real characters have real emotions, and know in their hearts what is right, even if they are a supposedly "evil" character. If you know the INTENTION of your character, you can bring that out through their drawings. I can tell just by looking at the warrior characters each of their own personalities, even though they look like "generic" warriors. The way they are standing, the expressions on their face, the COVERING of their face, all of these things show the true reality of a character.
I'd say that since you have the plot worked out, do some delving into the soul of each character individually. Ask some of these questions about each character:
What do they want in the end?
How would the character react to a close friend betraying them? Would they disown them? Would they try to get to the root of why they betrayed them?
Why is this character important to the story?
What skills do they have? More importantly, why did they choose to develop these skills? Did the need to pick up a sword arise from desperation for a loved one? Status symbol? Money? All of these are valid, and nothing is less than another. The point that matters most is making your character REAL. Real people have real INTENTIONS.
INTENTION!!!
http://www.writingforums.com/writing-101/62640-characterization-corner.html
This may help more than I can. Enjoy!
