Why do you feel the need to relate to a fictional character? People fell in love with Roxas and Riku because they were cool, badass characters back in the days. That was essentially it. People loved the gameplay prospects they brought that wasn't just playing as Sora.Because it makes them relatable. Relatability is the key to telling a good story. Why do you think so many fans fell in love with Roxas, or with Riku, or Xion? People grow attached to characters that they can sympathize with, and the best way to get players to sympathize with characters is to give said characters depth and personal struggles.