Kuro in an hybrid created in a laboratory, made from the desire to remove the natural weakness Charmanders have due to the flame of their tails. After an accident, he managed to escape and found a trainer, yet, after so much effort and evolving to his final stage, Kuro was abandoned. Due to that he became bitter and resents humans very deeply.
Squall on the other hand is a complete sweetheart. Born odd colored he had to take care of himself since young age and became very loyal to his trainer. He's the one that stands by Kuro's side the most and is always willing to help him vent out all the injures he bears in his heart due to being abandoned.