Soulmate Marriage is a unique feature that allows you to marry a player and gain abilities when playing together.
How to become Soulmates
Go to NPC [Soulmate] Cupid in Junon Polis to start the quest.
You can find a complete guide for the quest here.
Upon completing the quest, both players will be teleported to the chapel where a short wedding ceremony will take place. When this has finished, your characters will be linked.
You can open the Soulmate window from the menu or by pressing ALT+L. In this window you can see the appearance and stats of your Soulmate, as well as your Soulmate Skills.
Soulmate bonds can be leveled up to learn special abilities. You can do this by forming a party together and slaying Dragons in Sky Dragon Isle. The passive soulmate abilities will only be effective when you are in a party with your soulmate.
You can separate from your Soulmate after one month by going back to [Soulmate] Cupid