Parabatai are a pair of Nephilim warriors who fight together and are closer than siblings, regardless of their gender. They are partners who would lay down their life for the other. Being ones' parabatai is not merely a matter of promising to guard each other. The pair must stand before the Council and swear and oath to lay down their lives for each other.
Not all Shadowhunters are required to have parabatai though, and it is actually less common to have them. A Shadowhunter has only eighteen years to find and choose a parabatai. Once older than that, the ritual is no longer open to the Shadowhunter.
Shadowhunters are also forbidden from falling in love with their parabatai. Most Shadowhunters don't know the reason, but it apparently stems from a dark Clave secret.
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