Chot
Find Chot during game play, then enter ATLUS as a password.

Ikonda
Find Ikonda during game play, then enter LIGHT as a password.

Lucifroz
Find Lucifroz during game play, then enter DARK as a password.

Pandora
Find Pandora during game play, then enter MEGAMI as a password.

Starhair
Find Starhair during game play, then enter TENSEI as a password.

Defeating Sythfer
Buy Magic Cross from the demon just beside the dark castle. Talk to him and he will say 'I can awaken your demon's hidden powers' then 'Select your guide'. There is a fine but it is worth it. Buy the magic for Gale (who should be in his final form), then go through the castle. When you get to Sythfer, just keep using Magic Cross. It rarely misses and does about 150 damage each time. It attacks eight times; for about 900 total damage each time. He changes form frequently and does not have a health meter. Just keep attacking with Magic Cross and your other best attacks. He should fall in about fifteen turns.

Almost Instant Join
Repeat pressing A and the demon will join.