Beat the game once and play the Ex Map in Extra Mode. They will join in the next battle.

Alvan (Class:Bishop)
Finish the Tower, 7F

Fado (Class:General)
Finish the Ruins

Guren (Class:Dragon Master)
Finish the Ruins, round 5

Heiden (Class:Forest Knight)
Finish the Ruins, round 5

Ismea (Class:Sword Master)
Finish the Tower

Keselda (Class:Hero)
Finish the Tower, 3F

Leon (Class:Necromancer)
Finish the Ruins three times

Olsen (Class:Paladin)
Finish the Tower, 6F

Serena (Class:Mage Knight)
Finish the Tower three times

Walter (Class:Wyvern Knight)
Finish the Ruins, round 7