If you die during a boss battle, there is a high chance of the king coming to help(You
play as the King to revive Sora). A guarunteed chance is when you fight Xaldin in Beast's
castle.

I think that's in Final Mix II or maybe even in 358/2 Days. Unless you really do want to
summon Mickey or something. When you fight Cerebus for the first time and "die" Mickey
comes and saves you. Happens during the fight with Xalden, too, like Sev said.