If you want the Attack/Defense/Magic of the spirits, visit the Leveling Page.
You start the game with the Spear, which can summon Moja. Moja will grow to be your most powerful spirit by the end of the game.
In the Slug's Cave, you will fight the Hugeslug. If you defeat it, you'll win the Slug, which is used to summon Hugeslug.
In the Howling Cave, you will happen upon the Rockwing, which will allow you to summon the Wing Man.
Mod's Island Cave houses the Crystal, which is used to summon the Giant.
In the Pyramid near Ratal, you will find the Magicaxe, which is used to summon the Dwarf.
You will fight Hugefrog in the Eastern Lake. Defeating it will net you the Frog, which summons Hugefrog.
Halfling, your first defensive spirit, is summoned by using the Red Ring, which is in Fon's Cave.
Within Silver Mountain Cave, you may find the S. Cup, which summons Centaur.
The Lamp, which summons Twana, is found in the Intro Cave. Twana is the most powerful magic spirit, learning a wide array of spells.
In Fon's Cave, you may come across the Rapier that summons Elf, who specializes in defensive magic.
You will find the Goldrock in Silver Mountain Cave, and it will summon Gnome. Gnome's spells are offensive-oriented.