Gemini is one of the constellations of the Zodiac, and as such, its sword's cross-section is a six-pointed star. It always comes in pairs, and is a rather difficult enemy to face due to high stats. It is best to hit them with Armor Break and Power Break to lessen the damage inflicted by Gemini, and increase the damage dealt to the enemy. Casting Protect at the start of the fight is a smart idea, and using any magic on them will be effective as well if the player uses Mental Break.
When Oversouled, having elemental resistant equipment and access to Reflect and Shell helps protect against their elemental attacks.