Cassandra Jill Warlock, often referred to as The Black Snake or Cassa as she's commonly known, is an Old Blood vampire who was once friends with Jiro Mochizuki and Alice Eve. However, Cassandra betrayed them and killed Alice but Jiro questions why Cassa, after killing Alice, guarded her ashes. It is later revealed that she is the Lord of the Warlock family and she is a reincarnation of Morgan the Witch. Her prestigious bloodline grants her the ability to shapeshift. She was also the first to be bitten by the founder of the Kowloon Children bloodline Adam Wong, and was branded a traitor. During a brief conversation with Jiro during the Kowloon Shock, she claims that her happiest moments were with Alice and Jiro.
Cassa is an extremely powerful vampire, capable of matching a fully-powered Jiro in combat while not having absorbed any blood herself in three months. Her primary weapons are a chained silver cross and a katana. While Cassa is a skilled swordswoman, Jiro always had the advantage over her when it came to swordplay and fighting. Even as a Kowloon Child, Cassa enjoyed playing jokes and teasing people. It is revealed in the end that Cassandra's goal all along was to resurrect the Kowloon King Wong and continue his bloodline. Like all vampires, the continuation of their own Kowloon bloodline is all that Cassandra cares about, and she is willing to make any sacrifice in order to ensure its survival. The few exceptions to this are the killing of Jiro and the ending of the Sage bloodline.