Canas: You can get him to join in the stage in which you have to talk to the pirate captain, Fargus. He is in one of the houses that you start next to. He is a Shaman, who upgrades into a Druid. When upgraded to a level 20 Druid, the highest he can get, he is basically untouchable. He is very strong, even with his lowest spell Flux. With Luna, he can defeat the morph of the Black Fang mage in one critical hit.
Erk and Priscilla: You can get Erk to join you for a second time by talking to him with Serra sometime in an early 10 chapter. Erk is extremely useful in the later parts of the game. If you go to the village south of where you get him to join, you can find Priscilla. She is a healer, and is another highly useful character, considering she rides a horse and can get to injured units faster. She is also required to get another character to join your game.
Farina: You can get Farina by giving her 20,000 gold. She is Florina's sister and is a Pegasus Knight. She has a good lance and a javalin. She gains levels quickly and will be at level 12 when you get her.
Fiora: She is Florina's sister. Get her to join by talking to her with Florina. She is also a Pegasus Knight. She appears on the first mission on Valor, or Doom Island.
Gietz: You can get Gietz by talking to him with Dart. He is a Fighter. He has a silver ax and a steel bow. He can kill Wyvrens easily.
Guy: He joins if you talk to him with Matthew. He apparently told Matthew that he owed him his life after being forced to do so in order for Matthew to give him some food. He will join, not willingly, but it does not matter.
Harken: You must not kill the Sage, Bishop, and Druid in the chapter with the Boss Kenneth. In nine turns he will appear. Talk to him with Eliwood for him to join.
Hawkeye: You can get Hawkeye to join by talking to him with your main character (Eliwood or Hector). He appears in a sandstorm a few turns later in the southeastern part of the desert in which you are supposed to help Pent defeat the desert bandits.
Heath and Rath: Another Wyvern rider. You will meet him by talking to him with Lyn in the same chapter in which Rath reappears. You can also get him to join by talking to him with Lyn or perhaps your main character (Hector or Eliwood).
Karla: You can get her in the side quest battle preparations. You will talk to her with Bartre. She attacks you, and if you survive, you get her on your team. She is Karal's sister and is a Swordmaster on level 5. She has a Woe Dow.
Karel: You can get Karel to join by talking to him with Lyn. He appears in the chapter in which you are attacking the Black Fang base in the mountains. You may have to wait a little while, but he eventually appears. Talk to him with Lyn and he joins.
An alternate way is on Chapter 25 defeat three promoted enemies by turn 9. Talk to Karel with Eliwood and he will join your team. He is a Swordmaster.
Legault: Go to the chapter with Darin as a Boss. Talk to him with Lyn and he will join your team.
Nino and Jaffar: If you talk to Nino in the chapter when you are supposed to defend price Zephiel, she will join you (main character). You now may be able to get Jaffar to join you by talking to him with Nino.
Pent and Louise: You do not talk to Pent and Louise to get them to join. They join a few chapters after you first meet them. In joining, Pent gives you a Heaven Seal, which you can use to upgrade one of your lords (other than the main character, Hector or Eliwood). Upgrading Lyn is recommended, as she can use a bow which is very useful. Pent is a very high official in Bern, and Louise is his wife. Pent is a Sage when you meet him, and he has Elfire, a very useful spell. Louise is a Sniper, and is also very useful.
Raven and Lucius: When liberating Caelin, you can get Raven and Lucius to join your cause. To get Raven, talk to him with Priscilla, who is his sister. Once done, you can then get Lucius to join you by talking to him with Raven.
Renault: Go to Chapter 30 and go to the farthest up village. Select "Visit" and he will join your team. He is a Bishop.
Vaida: Vaida is a Wyvern rider who joins you if she survives in the first chapter that you encounter her. You later get her to join by talking to her with either Lyn or the main character of your path (Hector or Eliwood).
Wallace: Get him to rejoin by talking to him with Lyn. He appears in Bern when you are fighting because he got lost on his way to Caelin, which happens often.