You MUST recruit Haleth BEFORE you attack Stormfist. After conquering Stormfist he will be gone forever.
Other missable Characters
-Wilhelm = you must recruit those 2 after getting a Victory+ against the Hatred Fortress. Well just get your HQ, take Viki in your party and recruit them. The latest point is after the Invasion of Doraat. They will be gone after the new Queen's Campaign
-Mueller (look @ Wilhelm)
-Richard = when you don't get Wilhelm and Mueller
-Oboro = when you don't recruit Oboro he will be gone after the new queen's campaign. And the penalty is VERY hard for not recruiting him.
So whatever you do, HELP Oboro in ALL the investigations as long as you are in rainwall. If you don't the recruitment will be complicated. When you still haven't recruited him after the new queen's campaign he will be gone forever.
Murad's earliest recruiting point is after the battle of Lelcar. When you still haven't recruited him after reclaiming Sol Falena, he will be gone forever.
-Faylen and Faylon = recruit them after the Sable Event saying: all you did was to clear your good name. When you choose the other option, they will return to their hide. If you chose the second option, just take Roy in your Party and return to the Mountains. Take the captured loot and return it to the townsfolk of Sable. Those 2 CANNOT recruited anymore after the Invasion of Doraat. So better hurry
-Killey = Trigger the event in Agathe Prison after freeing Lucretia Merces and let him go. Then be nice to him when he comes to your HQ. Then allow him to join your party while you explore the Twillight Forest. After beating the boss ask him to join.
-Byakuren = he encounters you when you leave your HQ by crossing the bridge on the left side. You can recruit ihm when you have Levi or Genoh recruited. After conquering Sol Falena he won't appear again.
-Genoh = If you miss to talk to Fuwalafuwalu, you'll never learn about him.
-Cathari = make sure you get her right when you come to Port Spinnacks. Talk to the people there and the on on the exit to Twillight Forest will tell you about the Harbor of Hershville. Then go there by boat and talk to Cathari. Return to Spinnacks and run over to the other side and she will wait for you. She will be gone forever after reclaiming Sol Falena.
-Alhazred = make sure you trigger the scene with him and the merchant in Rainwall. If you don't trigger it he will be gone and you won't be able to find him again.
Did I forgot someone? If yes, tell me
Just my 2 potch