According to Tetsuya Nomura, he said that he did not expect the side characters from 358/2 Days and Birth by Sleep (i.e. Roxas, Xion, Axel, Saix, Terra, Ventus, Aqua, etc.) to be incredibly popular among fans, especially compared to the main reoccurring cast.

"I really didn't think that the characters from these two titles would become this popular. I had thought that players wanted to see more of characters like Sora or Riku. It was kind of unexpected that the characters from these two titles were so well received. [...] So, if possible, it would be great to include more of them or continue to share more of their story in future opportunities."
Contributed by PirateGoofy