Eidolons are the summons of Final Fantasy IX, they have the same properties from basically all of the Final Fantasies before 9 and they are different for Garnet and Eiko. Both characters learn and master Eidolons from specific stones and they have to equip it until they do master it, at which time they recieve the necesssary amount of AP. Garnet and Eiko can both summon Eidolons, however they have different Eidolons they can summon, Garnet has more offensive Eidolons than Eiko. Hers focus mainly on supporting the party, despite that, both Garnet and Eiko have summons that make it hard for anyone to decide on whether which one should be in their party.

Garent's Eidolons
Eidolon	Attack	Learned	MP Cost	Effect
Ramuh	Judgment Bolt	Peridot	22	Thunder Elemental Damage
Shiva	Diamond Dust	Opal	24	Ice Elemental Damage
Ifrit	Flames Of Hell	Topaz	26	Fire Elemental Damage
Atomos	G-Force 199	Amethyst	32	Non-Elemental Damage
Odin	Zantetsuken	Dark Matter	22	Non-Elemental Damage + Death
Leviathan	Tsunami	Aquamarine	42	Water Elemental Damage
Bahamut	Mega Flare	Garnet	56	Non-Elemental Damage
Ark	Eternal Darkness	Pumice	80	Shadow Elemental Damage
Eiko's Eidolons
Eidolon	Attack	Learned	MP Cost	Effect
Carbuncle	Ruby Emerald Pearl Diamond Light	Ruby Emerald Moonstone Diamond	24	Casts Reflect / Other Spells
Fenrir	Terestrial Rage Millennial Decay	Sapphire Maiden Prayer	30	Earth/Wind Elemental Damage
Phoenix	Rebirth Flame	Use Phoenix Pinon	32	Fire Elemental Damage + Revival
Madeen	Terra Homing	Ribbon	54	Holy Elemental Damage