You must defeat the Black Knight in chapter 27 going on to 28. You fight him and Mist comes with Ike, but she needs to be promoted by this point, or you'll have a difficult time. Mist has to be able to kill the healers (Bishops) and the one soldier that appear to assist the Black Knight. You must be able to defeat the Black Knight in five turns or less or else Nasir will not be available for the rest of the game.

Ike should ideally be at level 20 or more. Have Mist promoted by at least by one level. Have Mist occupy the Black Knight's flunkies and focus your heaviest hitters on the Black Knight. Good luck.