Assassin is a martial name currently held by a member of the Calamities. Virtually nothing is known about Assassin other than his gender, and a great personal love for killing people in the most creative way possible.
For obvious reasons, he goes to extreme lengths to keep his appearance, name, background and intentions hidden. He is also the only one of the Calamities to not openly state that his loyalties lie with Amadeus over the Tower, although it is impossible to extrapolate from this.
He has appeared on screen as of Dues (extra chapter of October). His appearance was described as "
☀ There was a silhouette in the dark she could almost make out the features of. It was a man. She blinked. It was a woman. It was neither and she couldn’t remember a single thing about either of the people she’d glimpsed. Not their hair, not the shape of their face not even the colour of their skin." We also have Word of God that he did appear at some point in Book I disguised as a background character.
He also appeared on the screen in Coda. His appearance was described as "
☀Every blink had a different face to it, and the longer I watched the less I could remember about any of them. "
As of Book V, it is rumored that he was killed by the Blue Mage after assassinating many of the members of the upper Tiers in the Thalassocracy of Ashur. However, Catherine remained doubtful of this, stating that such rumors had circulated before, and had always been proven incorrect in the end.
As of book VII, it is revealed by Scribe that Assassin is a product of one of her Aspects: by inscribing certain Words on a body (living or dead), she can force the body to behave in certain ways. When done to a corpse, if retrieved, the Words evolve, becoming more and more Named with each passing iteration. The more advanced versions require powerful corpses, preferably Named.