Complex sentence: Ranma has repeatedly been unable or unwilling to accept defeat, regardless of how outmatched he is, or how irrational this may turn his behaviour, although his confidence can be shaken enough to not be able to use the Moko Takabisha.
Split into simpler sentences: Ranma has repeatedly been unable or unwilling to accept defeat, regardless of how outmatched he is, or how irrational this may make his behaviour. Despite this, his confidence can be shaken enough to not be able to use the Moko Takabisha.