Unlocking each gaiden stage, or side quest, must be done in the chapter of the same number (i.e. Lyn's chapter 7 gaiden must be unlocked by meeting the listed requirements in chapter 7). The only exception to this is Hector's gaiden 19xx, which has an additional requirement.
Unlockable
How to Unlock
Eliwood Chapter 13x
Visit the village in the top left-hand side of the map.
Eliwood Chapter 16x
Keep at least one of the NPC soldiers alive.
Eliwood Chapter 18x
Finish chapter 18 in 15 turns or less.
Eliwood Chapter 22x
Recruit Hawkeyes and gain 700XP or more with your entire party. Each member's XP gain contributes to this total.
Eliwood Chapter 26x
Recruit Nino and have her Talk with Jaffar.
Eliwood Chapter 29x
No special requirements.
Hector Chapter 13x
Visit the village in the top left-hand side of the map.
Hector Chapter 17x
Keep at least one of the NPC soldiers alive.
Hector Chapter 19x
Finish chapter 19 in 15 turns or less.
Hector Chapter 19xx
Before chapter 11, train Nils to at least level 7 and kill the magic barrier in chapter 19x.
Hector Chapter 23x
Recruit Hawkeyes and gain 700XP or more with your entire party. Each member's XP gain contributes to this total.
Hector:Get an A support w/ either Florina, Farina, or Lyn before beating the game. Eliwood:Get an A support with either Ninian, Fiora, or Lyn before beating the game.
To obtain Harken on Jerme's version of Pale Flower of Darkness, do not unlock too many doors before turn 11. When Harken appears, Talk to him with either Eliwood, Oswin, or Marcus.
To obtain Karel on the same map, unlock more than 5 doors by turn 11. This is very difficult, and this level is recommended for getting Harken only. If you DO get Karel, talk to him with your main lord (Eliwood in his story, Hector in his).
To obtain Harken in Kenneth's version of PFoD, kill fewer than 3 promoted magic enemies by turn 11, and talk to him with Eliwood, Marcus, or Oswin.
To obtain Karel on Kenneth's version of PFoD, kill more than 3 promoted magic units by turn 11. Talk to him with your main lord.