I hated Hollow Bastion, but mostly because of the Riku fight. I rushed through everything else and was pitifully equipped when I finally got there. It took forever to beat him.
Only once. He helped me in Beast's Castle during the battle with Xaldin. I was surprised. I thought I had died, so I threw the controller down. Then I realized I was Mickey and jumped up, so relieved. That's the only time I remember him helping.
Hades - "Did you forget who you're talking to?! I am the Lord of the Dead!"
Auron - "No wonder no one wants to die."
Riku - "Well, there is one advantage to being me... Something you could never imitate."
Sora - "Really? What's that?"
Riku - "Having you for a friend."
I'd want Aerith, Tifa, or Yuffie definitely. It would be reminiscent of my days playing FF7 with my three favorite party members.
Rikku and Paine from FFX/X2 (Not in fairy form) would be awesome too.