She is the "alternate identity" of Hanekawa Tsubasa which is manifested when the weirdness called "sawarineko" takes control of Hanekawa's body. She entered in Hanekawa's body and mind after she had buried with dignity a dead cat that Hanekawa found on the street when she was walking along with Araragi.
Black Hanekawa began as a sawarineko which was found lying dead on the road. On May 29, Tsubasa, that time accompanied by Koyomi Araragi, finds the cat and buries it on a nearby patch of ground.
That night, she attacks Tsubasa and possesses her body, creating her "alternate identity". Days later, Black Hanekawa began a string of attacks against people at night, whose energy was drained away by Black Hanekawa's ability as a sawarineko.
Unlike Hanekawa Tsubasa, who is soft-spoken and has a reserved personality, Black Hanekawa possesses a completely different personality and appearance. She is playful and talkative, her words often laced with the sounds a cat often makes, "nya", and is sharp-tongued and blunt when talking to someone.
However, she also has a more brutal side which, when combined with her superhuman strength and her distracting choice of clothing, makes her a rather frustrating and difficult opponent to face. She is also known to be a manipulative mind, faking death to attract its next victim. This manipulative character was further strengthened by Tsubasa's knowledge, which she was able to gain access to.
Surprisingly, the sawarineko which takes over Tsubasa's body has developed a degree of responsibility for Tsubasa and has revealed that she "is inclined to help her" after understanding Tsubasa's problems at home. This implies a strong bond between Tsubasa, who was revealed to actually permit the sawarineko possessing her body, and the sawarineko, who has gained a vast amount of knowledge due to taking over Tsubasa's body, something which is completely unheard of when it comes to encounters with a sawarineko, which is known for its ruthlessness to its victims.