So, a new member was asking how to unlock Human Idoun in Fire Emblem 6 (http://serenesforest.net/forums/index.php?showtopic=12136) and as a good member that I am, I shall help out.
To unlock Human Idoun, you must have beaten the game on both easy and hard mode at least one time each. Now, playthrough Hard Mode again and this time around, you MUST use every character at least once in the game. This may sound easy, however, the catch is that you can NOT have any unit killed and you can not reset the game. If you do reset the game, you might as well start a whole new game.
You must complete ALL the side quests, Gaiden chapters, and you must get to the last chapter in under 45 hours. Only Roy or Fa can fight Idoun, DO NOT USE ANY OTHER UNIT TO BATTLE DRAGON IDOUN IN THE LAST CHAPTER!!! She'll go berserk and take it out on everyone in your party, killing them with amazing OHKOs.
If you were able to defeat Dragon Idoun with Roy, you unlock a hidden chapter in which you are on a map with Roy and Idoun only. This map is a beautiful flower field in a quiet, secluded forest. There is no music, no words. Human Idoun and Roy look at each other and go into combat.
If you are able to defeat Idoun in under three rounds, you unlock her.
Hope I could been of help!