There is actually another way to keep Jill other than what is already here. The alternate way is for her to have a Support level A with Lethe (through the base). Take note that you SHOULD be doing ALL Support conversations after finishing each chapter.
Ashanard
Successfully complete the game fifteen times to unlock Ashanard when playing the bonus trial maps.
Astrid
In the mission where you are supposed to protect the Apostle's ship in Chapter 13, there is a Bow Knight named Astrid. Have Ike talk to her and she will join your team. She has the Paragon skill which makes her gain lots of experience points.
Brom
In the "Prisoner Release" mission in Chapter 10, open Brom's cell door. Have Ike talk to him and he will join your team. He is in the same room as Kieran. He is average, but not that great towards the end.
Bryce
Successfully complete the game ten times to unlock Bryce when playing the bonus trial maps.
Calill
Read the conversation before Chapter 20.
Character Unlocks
Hunter again from all you fans of Metroid Prime 2 now to Fire Emblem.
CH09-Mist-Automatically joins
Rolf-Automatically joins
Marcia-Talk with Ike
CH10-Lethe-Automatically joins
Mordecai-Automatically joins
Volke-allow him to join your team at the start of the Chapter
Nephenee-Talk with Ike
Brom-Talk with Ike
Kieran-Talk with Oscar
CH11-Talk with Mordecai
CH12-Sothe-Allow him to join your team
Jill-Talk with Ike
CH13-Astrid-Talk with Ike
Gatrie-Talk with Astrid
CH14-Makalov-Talk with Marcia
CH15-Stefan-Move either Lethe of MOrdicai to the Varg Katti space
Muarim-Clear the Chapter
Tormod-Clear the Chapter
CH16-Devdon-Talk with Soren
CH18-Reyson-Go to the First Report, complete Chapter 17, and Accept his halp in Chapter 18
Janaff-Automatically joins
Ulki-Automatically joins
Tanith-Automatically joins
Shinon-19, talk with Lofa in Chapter 18, then kill him with Ike (my friend could be wrong)
CH20-Calill-Go to "Mysterious Girl" report before doing the Chapter
CH21-Tauroneo-Talk with Ike, Mist, or Soren
CH23-Ranulf-Automatically joins (finally)
Haar-Talk with Jill
CH24-Lucia-Automatically joins
Bastian-Automatically joins
Geoffrey-Must survive Chapter 24, will join on Chapter 25
CH25-Largo-Go to the "Mysterious Man" Report before the Chapter
CH26-Elincia-Automatically joins
CH28-Nasir-Will join automatically if you do not defeat the Black Knight in Chapter 27
CH29-Tibarn, Naesala, and Giffca-Choose all of them during the beginning of the Final Chapter.
Devdan
Devdan is a Halberdier who is indebted to Duke Tanas. In the mission where you are sent to investigate Tanas's in Chapter 16, make sure to bring Mist, Rolf, Soren, or Ilyana. Devdan will attack anybody except children. Have one of these four characters talk to him and he will join your team.
Ena
Ena will join your party if Dark Knight is not killed in Chapter 27.
Gatrie
Gatrie leaves you around Chapter 8, but returns later in the mission where you must protect the Apostle's ship in Chapter 13. After getting Astrid, have her talk to Gatrie and he will join your team. You will now have your wall back.
Geoffrey
If Geoffrey survives in Chapter 24, he will join your party.
Haar
In Chapter 23, speak to Haar with Jill.
Ilyana
In Chapter 8: Despair & Hope, there is a female mage, nine spaces north and two spaces west, named Ilyana. Have Ike talk to her and she will join your team. She has Elithunder as her attack.
Ilyana
On Chapter 8: Despair and Hope, there is a female mage, nine spaces north and two spaces west, named Ilyana.If you talk to her with Ike, she will join your team. She has Elithunder as her attack.
In-game reset
While playing a game, hold X + B + Start.
Jill
In Chapter 12, Jill joins you automatically on the way to Bengion. She is a Wyvern Rider, your first, or perhaps second, flying unit. She stays with you all the way until you invade Daein. If you want to keep her, make sure you do not use her in any battles in Daein.
Kieran
In the "Prisoner Release" mission in Chapter 10, have Oscar talk to Kieran and he will join your team. Kieran is an Axe Knight.
Largo
Read the conversation before Chapter 25 to get Largo.
Lethe
Have Lethe survive Chapter 9.
Lose Shinon and Gatrie
In Chapter 8: Despair & Hope, after Ike's father dies, Shinon and Gatrie will leave thinking that Ike is not a capable leader.
Makalov
In Chapter 14, after the Apostle gives your first mission, bring Marcia. In the top left of the screen is a Sword Knight named Makalov. He is Marcia's brother. Have Marcia talk to him and he will join your team.
Marcia
In Chapter 3, make sure Ike talks to the Pegasus Knight at the top left of the screen before she gets killed. Do not worry too much; she dodges a lot. She will mumble something about repaying a debt and flies off. If you do this, later in Chapter 9 she will return about two to four turns into the mission. Have Ike talk to her and she will join your team. She is not the greatest, but with a bit of work turns into a very reliable unit. Plus, she flies.
Mia
In Chapter 7, after a few turns, a charter named Mia will walk on screen from the East Exit. Have Ike talk to her and she will join your team. She has a sword.
Mordecai
Have Mordecai survive Chapter 9.
Nasir
Nasir will join your party if Dark Knight is not killed in Chapter 27.
Nephenee
In the "Prisoner Release" mission in Chapter 10, open Nephenee's cell door. Have Ike talk to her and she will join your team. Nephenee is a soldier and can become one of your better units.
Oliver
Successfully complete the game three times to unlock Oliver when playing the bonus trial maps.
Petrine
Successfully complete the game seven times to unlock Petrine when playing the bonus trial maps.
Reyson
Read the Conversation before Chapter 18, complete Chapter 17, then accept his assistance in Chapter 18.
Shihiram
Successfully complete the game five times to unlock Shihiram when playing the bonus trial maps.
Sothe
Read the conversation before Chapter 12.
Sound Room option
Successfully complete Story mode to unlock the "Sound Room" option.
Stefan
In Chapter 15, move Lethe or Mordecai to the upper right part of the map.
Tauroneo
In Chapter 21, speak to Tauroneo with Ike, Soren, Mist, Sothe or Tormod.
Theater option
Successfully complete Story mode to unlock the "Theater" option.
Triangle attack
This is a special attack that can be done with Boyd, Rolf, and Oscar. They all must be able to use a bow, meaning Boyd needs to be a warrior and Oscar must be given the ability to use bows. Form a triangle around the target and choose one of the brothers to do the attack. Note: You must see two conversations between the three at your base. Make sure to check the "Info" option frequently.
View chapter dialogue
Press Z while someone is talking to see all previously viewed dialogue for that chapter.
View game script
While a character is talking, press Z to see all previously viewed dialogue for that chapter.
Volke
Hire Volke at the start of Chapter 10.
Zihawk
In Chapter 11, speak to Zihawk with Lethe or Mordecai and he will join your team.