Aran
Part 1, Chapter 3: Speak to him with Laura.
Calill
Part 2, Final: View the info conversation before 2-F.
Devdan
Part 2, Chapter 3: View the info conversation before 2-3.
Fiona
Part 1, Chapter 7: Joins automatically if she survived 1-6.
Gareth
Part 4, Final: Joins automatically if he survived part 3 of the final chapter.
Haar
Part 2, Prologue: Speak to him with Marcia.
Heather
Part 2, Chapter 1: Speak to her with Brom or Nephenee.
Janaff
Part 3, Chapter 8: Speak to him with Ike or Reyson.
Lehran
Part 4, Final: Joins automatically after viewing Ike's memory scene (second playthrough or higher)
Meg
Part 1, Chapter 4: View the info conversation before 1-4.
Muarim
Part 1, Chapter 7: Joins automatically when Tormod is recruited.
Nasir
Part 4, Final: Joins automatically if he survived part 3 of the final chapter.
Oliver
Part 4, Chapter 4: Speak to him with Rafiel.
Pelleas
Part 4, Chapter 2: Joins automatically if he survived 3-13 (second playthrough or higher)
Renning
Part 4, Final: View the info conversation before 4-F.
Stefan
Part 4, Chapter 3: Move Micaiah onto the tile 11 up and 1 accross from the bottom right tile.
Tormod
Part 1, Chapter 7: Speak to him with Sothe.
Ulki
Part 3, Chapter 8: Speak to him with Ike or Reyson.
Vika
Part 1, Chapter 7: Joins automatically when Tormod is recruited.
Volke
Part 4, Chapter 5: Pay him 3000G with Bastian or Elincia.
Volug
Part 1, Chapter 5: View the info conversation before 1-5.