Way of the Gemini monks fight with a Soul Ally, a spiritual manifestation of their soul which is projected into the material plane. This Soul Ally allows the monk to attack enemies at a distance when they use their Flurry of Blows feature and as the level, the Soul Ally can use Stunning strike, switch places with the monk, and even eventually use the Flurry of Blows feature itself.

Soul Ally

At level 3, the monk after training their body and mind to be able to contact their soul ally, can call upon them to aid them in battle. As an action, you may spend 1 ki point to summon your Soul Ally in an unoccupied space within 10 feet of you. The Soul Ally lasts for a number of minutes equal to your monk level. If the Monk is incapacitated or dies, the Soul Ally disappears. 

The Soul Ally shares the same stats as you and it shares your movement speed. It also shares the same turn as you. Your Soul Allies health is equal to half of your hit point maximum rounded up and has an AC of 12 + your dex bonus. It can understand all languages you know and can telepathically communicate with the monk, but may not physically speak itself. If your Soul Ally moves more than 30 feet away from the monk, the Soul Ally will disappear.

When you use your Flurry of Blows feature, you may use one of your attacks to allow your Soul ally to make an unarmed attack. Your Soul Allies damage dice is your martial arts die. Also as a bonus action, you may have your Soul Ally provide the help action to any creature that is within 5 feet of the Soul Ally.

Stunning Soul

Beginning at level 6, when your Soul Ally successfully hits an unarmed strike, you may spend 1 ki point to force the target to make a Constitution saving throw. On a failed save, the target is stunned until the end of your next turn. In addition your Soul Allies unarmed attacks are considered magical for the purpose of overcoming resistances and immunity to nonmagical attacks and damage.

Spiritual Bond

Your bond with your Soul Ally has strengthened over the course of your journeys, allowing you to briefly step into the Astral Plane to switch places with your Soul Ally. As a reaction when you would take damage, you may switch places with your Soul Ally and have it take the damage you would have taken for you.

One with the Soul

At 17th level, you may spend 1 ki point to have your Soul Ally use the flurry of blows feature after it hits a melee unarmed strike.

In addition when you use your Spiritual Bond feature, your Soul Ally can make an unarmed attack if it takes damage.