I had a thought that Imu might actually have been Joyboy's "Usopp". Luffy and Usopp's argument and fight in Water 7 is supposed to represent a conflict that happened between Joyboy and Imu 900 years ago, that led to Imu going against his former captain, and ultimately defeated him, because he knew his weaknesses and fighting style. Usopp fights in an unorthodox way, using trickery, deception, and various weapons to exploit Luffy's weaknesses and personality, and that might be exactly how Joyboy was originally defeated. Imu ultimately let his insecurities and pride beat his loyalty to Joyboy, and Usopp will do the opposite.