Echidna
To get Echidna in your party, select the "You're about to find out!" option when facing her. Win the battle that starts with an overkill (defeated and not kneeling on ground). When you face Echidna the second time, make sure that Spreo has his final, Samurai, rank by getting his legendary weapon. Once again, defeat Echidna with an overkill.

Jade
To get Jade in your party, defeat him with an overkill the first time you face him, and defeat all of his men. Also win by an overkill in the second battle against him. Then, select the "I wasn't really going to kill him" option when it becomes available. In Chapter 4 or 5 get the quest that involves guarding a gold transport. Jade will be there and flee. Go to the guild and look for a letter from Jade. Fight him and he will join your party.

Tia
To get Tia in your party, do not let her die in any battle that she is in except for the first golem battle. If she dies there, she will appear in a different location. Also, read all of her letters. After getting the twelfth letter she will appear in Silva or Aphen.

Viser
To get Viser in your party, defeat him with an overkill during the first battle and kill all of his golems. During your second battle with him, also win with an overkill. You must also overkill all Bosses that run away.

Easy experience
Go to the catacombs in any level (level 1 recommended). Have all the characters who you want to gain experience for and a healer (required). When the battle starts, attack your own teammates. After the attack, your character will gain experience. The higher the character's level that you will attack, the better. You can either heal the damaged character (for experience for the healer) or kill him or her (much more experience will be gained). Note: If your characters are killed, their Luck status will be reduced.