Volke

Best answer as chosen by Asker / Ratings
Unlike in Path of Radiance, the majority of the characters in this game are automatically recruited. Here is a list of those who are not:

Aran (Part 1, Chapter 3) - Talk to him with Laura.

Meg (Part 1, Chapter 4) - Read Info conversation at base.

Fiona (Part 1, Chapter 7) - Must survive Chapter 6-2.

Tormod, Muarim and Vika (Part 1, Chapter 7) - Talk to Tormod with Sothe. All three will be recruited at the same time.

Haar (Part 2, Prologue) - Talk to him with Marcia.

Heather (Part 2, Chater 1) - Talk to her with Brom or Nephenee.

Danved (Part 2, Chapter 3) - Read Info conversation at base.

Calill (Part 2, Endgame) - Read Info conversation at base.

Janaff and Ulki (Part 3, Chapter 7) - Talk to one of them with Ike or Reyson and both will be recruited. Will automatically join in Chapter 8 if both are not recruited in Chapter 7 but still survive the battle.

Peleas (Part 4, Chapter 2) - Joins automatically at the start of turn 2 if you chose to sapre him in Part 3, Chapter 13. This option is only available from the second playthrough and onwards.

Stefan (Part 4, Chapter 3) - Place Micaiah 1 square left and 12 squares up from the most bottom-right tile. You can do this with Lethe or Mordecai if you have transferred Path of Radiance data over to Radiant Dawn.

Oliver (Part 4, Chapter 4) - Place Rafiel into his movement range. He will walk over to Rafiel and automatically be recruited after a short conversation.

Volke (Part 4, Chapter 5) - Pay him 3000G with Elincia or Bastian.

Renning (Part 4, Endgame 1) - Read Info conversation at base.

Nasir (Part 4, Endgame 4) - Will join automatically if he survives Part 4, Endgame 3.

Gareth (Part 4, Endgame 4) - Will join automatically if he survives Part 4, Endgame 3.

Lehran (Part 4, Endgame 5) - As with Pelleas, he can only be recruited in the second playthrough and onwards. Deploy the Black Knight in Part 1, Endgame. Have Ike fight the Black Knight in Part 3, Chapter 7. If you meet these requirements, Lehran will automatically join you at the start of Part 4, Endgame 5.

You can also recruit Jill (Talk to her with Haar), Zihark (Talk to him with Lethe or Mordecai) and Meg (Talk to her with Brom) from Micaiah's team into Ike's team in Part 3, Chapter 7 and Part 3, Endgame. You're probably not using Meg, but I do not recommend that you do this in Chapter 7 as Jill and Zihark are two of the few good units in Micaiah's party. It doesn't matter in Endgame since you can place them wherever you like afterwards. In fact, you can recruit them in Endgame so they can gain a few extra points.